Geological steering method for horizontal wells in bioherm gas reservoirs

Abstract

This invention discloses a geological steering method for horizontal wells in bioherm gas reservoirs. The method involves data collection, selection of standard wells, development of stratigraphic positions, model building, calculation of well inclination, positioning of the drill bit, and tracking and controlling the drilling trajectory to reach and penetrate the bioherm. A trajectory control method is established before encountering the bioherm, forming a trajectory adjustment mode for the target entry stage based on the principles of "controlling well inclination, cutting vertical depth, exploring the Changxing Formation, and finding dolomite." This reduces the risk of missing the bioherm due to excessive well inclination above it or insufficient well inclination below it, thus solving the problem of low bioherm encounter rates.

Description

Technical Field

[0001] This invention belongs to the field of natural gas development technology in bioherm shoals, specifically relating to a geological steering method for horizontal wells in bioherm shoal gas reservoirs. Background Technology

[0002] The bioherm gas reservoirs in the Changxing Formation of the Sichuan Basin are mostly ultra-deep, low-porosity, low-permeability, bottom-water, high-sulfur, medium-carbon dioxide, low geothermal gradient, normal pressure, structural-lithological, and carbonate gas reservoirs. These reservoirs exhibit the following characteristics: First, the reservoirs are deeply buried, generally exceeding 4000.00 m; second, the bioherms are small and dispersed, with complex spatial distribution characteristics, significant variations in horizontal and vertical distribution, strong reservoir heterogeneity, and complex structural-lithological features; third, key geological control points are not obvious, seismic marker beds are weak, and lithological interfaces and well logging response characteristics are not clear.

[0003] The unique geological characteristics of bioherm gas reservoirs present the following technical challenges to the geological steering of horizontal wells: First, the thickness of the Feixianguan Formation above the Changxing Formation varies greatly, the closer to the target layer, the fewer the geological control points, and the more difficult it is to accurately predict the target layer. It is also difficult to accurately drill into the target geological body, and there is even a risk of missing the target. Second, the vertical thickness distribution of the gas reservoir varies greatly, and the lateral heterogeneity is strong. It is difficult to maintain a trajectory to penetrate high-quality reservoirs for a long time, and the drilling rate of high-quality reservoirs is difficult to guarantee.

[0004] The journal *Drilling and Production Technology* (Vol. 37, No. 4) published a key geological steering technology for ultra-deep horizontal wells in complex reef and shoal bodies. It documented that the development of the high-sulfur gas reservoir in the Changxing Formation of the Yuanba gas field was primarily achieved using extended-range horizontal wells. Addressing the limitations of conventional geological tracking analysis techniques in meeting the requirements for long-distance penetration of high-quality reservoirs in complex reef and shoal bodies, this paper, based on the characteristics of the gas reservoir, comprehensively applies new technologies such as X-ray fluorescence analysis, nuclear magnetic resonance analysis, and near-wellbore constrained inversion to study challenges in carbonate strata identification, reservoir evaluation, and reservoir prediction during drilling. Standards for elemental logging lithology identification and nuclear magnetic resonance property analysis were established, resulting in a practical and effective geological comprehensive steering and trajectory optimization technology for ultra-deep horizontal wells in the Yuanba area. However, the difficulty of long-distance penetration of high-quality reservoirs leads to a lower encounter rate.

[0005] CN106869790B discloses a rapid and precise geological steering method for shale gas horizontal wells, including data collection, identification of steering comparison standard wells and reference wells, determination of steering marker layers and target prediction schemes, establishment and adjustment of two-dimensional and three-dimensional geological steering models of the well area, and timely correction of the two-dimensional vertical profile and three-dimensional geological steering model of the well to be steered; using the lithology, well inclination, and natural gamma ray while drilling characteristics of the well to be steered compared with the standard well and reference well, stratigraphic correlation is performed, and three-dimensional geological steering model is adopted. Figure 1The system can quickly achieve geological steering of shale gas horizontal wells and effectively solve the geological steering problem. However, it is not suitable for geological steering of bioherm gas reservoirs with strong reservoir heterogeneity and complex tectonic-lithological structures.

[0006] CN112983275B discloses a geological steering trajectory control method for horizontal sections of continuously undulating reservoirs in shale gas wells. The core of this method is to set marker points at the undulation points of the target layer's top and bottom interfaces, and combine these with the entry and exit points to form an optimal trajectory control path. Using appropriate formulas, the well inclination angle required to reach the entry point and the well inclination angle required to reach the next point in each segment are obtained, thus achieving geological steering trajectory control for horizontal sections of continuously undulating reservoirs. This invention offers a simple control method, optimizes engineering curvature, reduces trajectory adjustment frequency, shortens the drilling route, and improves drilling quality and timeliness. However, it is not suitable for geological steering of bioherm gas reservoirs with indistinct key geological control points, weak seismic marker layers, indistinct lithological interfaces and logging response characteristics, strong heterogeneity, and complex tectonic-lithological structures. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a geological steering method for horizontal wells in bioherm gas reservoirs that can improve the problem of low drilling encounter rate in bioherm gas reservoirs in the prior art.

[0008] To achieve the above objectives, the present invention provides a geological steering method for horizontal wells in bioherm gas reservoirs, specifically including the following steps:

[0009] 1) Collect geological data of drilled wells in the well area, as well as design data and geophysical data of horizontal wells being drilled;

[0010] The design data for drilling horizontal wells includes drilling geological design and drilling engineering design. The drilling engineering design includes wellbore trajectory design data and target point data.

[0011] Geophysical data for horizontal wells being drilled includes geological structure maps and seismic profiles along the wellbore trajectory.

[0012] 2) Based on the geological data of existing wells in the well area, select the vertical wells that encounter bioherm bodies as standard wells;

[0013] Based on the data of bioherm bodies encountered in the well area and the geological design of the horizontal well being drilled, the development strata of the bioherm bodies in the horizontal well being drilled were determined.

[0014] 3) Select abrupt change points in the natural gamma curve morphology of standard wells as marker points;

[0015] Based on the collected geophysical data of the horizontal well and the development strata of the bioherm bodies in the horizontal well, an initial model of the distribution of bioherm bodies along the trajectory of the designed horizontal section of the horizontal well was established.

[0016] 4) Calculate the well inclination segment control chart corresponding to each marker point above the bioherm body of the standard well and the wellbore trajectory design data of the horizontal well being drilled.

[0017] 5) While drilling, create a formation comparison map between the horizontal well being drilled and the standard well to locate the current drill bit position;

[0018] 6) Drilling while tracking the control trajectory to reach and penetrate the bioherm body in the horizontal well being drilled.

[0019] Furthermore, in step 1), the geological data of the drilled wells in the well area includes formation lithology, drilling time, total hydrocarbons, methane, natural gamma, elemental logging data, and rock thin section data.

[0020] Furthermore, in step 2), based on the geological data of the wells already drilled in the well area, the vertical well that is closest to the horizontal well being drilled and has encountered a bioherm is selected as the standard well.

[0021] Furthermore, in step 3), based on the formation lithology and natural gamma characteristics of the standard well determined in step 2), the abrupt change characteristic point of the natural gamma curve morphology is selected as the target control marker point for drilling horizontal wells.

[0022] Furthermore, in step 3), based on the collected geophysical data of the horizontal well and the development strata of the bioherm body in the horizontal well, the geological guidance software is used to establish an initial model of the lateral and longitudinal distribution of the bioherm body in the horizontal section trajectory direction of the horizontal well, to characterize the size and lateral and longitudinal distribution characteristics of the bioherm body in the horizontal well, and to predict the thickness and depth of the bioherm body in the designed horizontal section trajectory direction.

[0023] Furthermore, in step 4), based on the vertical depth of each marker point, the vertical thickness between each marker point, and the vertical thickness from each marker point to the designed target point A, combined with the wellbore trajectory design data and the designed entry angle, the well inclination corresponding to the control point is calculated from bottom to top, starting from the vertical depth of the bioreef body in the horizontal well being drilled, and the well inclination segment control chart corresponding to the marker point above the bioreef body in the horizontal well being drilled is established.

[0024] Furthermore, the specific process of step 5) is as follows:

[0025] 51) Real-time collection of lithology, gas logging, natural gamma ray, elemental logging, and wellbore trajectory data for horizontal wells being drilled;

[0026] 52) The collected data will be processed into a vertical well using geological drawing software, and a vertical depth comparison chart of the formation between the drilled horizontal well and the standard well will be drawn.

[0027] 53) Locate the current drill bit position based on the marker point above the bioherm body of the horizontal well determined by comparing it with the standard well.

[0028] Furthermore, the specific process of step 6) is as follows:

[0029] 61) Trajectory control before target entry is achieved by locating and tracking the well inclination segment control chart corresponding to the marker point;

[0030] 62) Determine whether the bioherm body has been reached based on the lithology, gas logging, drilling time, and elemental logging characteristics of the standard well bioherm body; drill at an angle of 10.0° to 15.0° to reach the marker point closest to the designed target point A of the horizontal well; if no bioherm body is found after reaching the designed target point A of the horizontal well, gradually increase the inclination to 83.0° to 87.0°, and probe for the bioherm body at an angle of 4.0° to 6.0°; if the bioherm body is reached, adjust the inclination and level the trajectory.

[0031] 63) By comprehensively utilizing lithology, gas logging, drilling time, elemental logging, cuttings thin sections, and natural gamma data while drilling, the bioherm distribution model is dynamically corrected on the initial model of bioherm distribution in the horizontal section trajectory direction of the horizontal well being drilled, and the drilling trajectory is optimized and adjusted to ensure the encounter rate of bioherms in the horizontal section.

[0032] Furthermore, the specific process of step 61) is as follows:

[0033] 61a) Based on the vertical depth reached when each marker point is encountered in the horizontal well and the corresponding formation thickness of the standard well, the vertical depth of the top of the bioherm body designed for target point A and the vertical depth of target point A are calculated point by point using the equal thickness method.

[0034] 62b) Based on the well inclination control chart corresponding to the marker point above the bioherm body established in step 4), and combined with the well inclination of the actual drilling trajectory of the horizontal well, compare the actual well inclination when drilling to each marker point with the well inclination required by the well inclination control chart of the marker point; if the well inclination error between the two is within -2.0° to 2.0°, continue drilling according to the designed trajectory; if the well inclination error between the two is outside -2.0° to 2.0°, adjust the build-up rate and redesign the trajectory so that the well inclination when drilling to the next marker point is consistent with the well inclination required by the well inclination control chart of the marker point.

[0035] The formula for calculating the slope rate is:

[0036]

[0037] In the formula, K—slope rate, ° / m;

[0038] α—The difference in well inclination angle between two adjacent marker points, in degrees;

[0039] H—the vertical depth difference between two adjacent marker points, in meters.

[0040] Furthermore, the specific process of the optimization and adjustment is as follows:

[0041] 63a) Use data collected while drilling to draw a directional composite chart. The directional composite chart includes sub-layer, lithology, total hydrocarbon, methane, and natural gamma ray while drilling. Elemental logging selects eight curves for aluminum, silicon, potassium, sodium, calcium, magnesium, barium, and strontium.

[0042] 63b) Based on lithology, total hydrocarbons, natural gamma ray during drilling, and elemental logging data on the directional composite chart, combined with cuttings thin section and well inclination data for real-time tracking and positioning, the initial model of bioherm distribution along the designed horizontal section trajectory of the horizontal well is dynamically corrected, and a bioherm horizontal section trajectory traversal map is generated during drilling. Model adjustment method: Calculate the difference between the vertical depth of the bottom interface of the bioherm at the actual drilling location and the vertical depth of the bottom interface of the initial model at the corresponding closure displacement. If the difference is positive, the initial model needs to be moved vertically downwards; if the difference is negative, the initial model needs to be moved vertically upwards. The magnitude of the model movement is consistent with the difference. A difference of 0 indicates that the actual drilling results match the initial model, and the initial model does not need to be adjusted.

[0043] 63c) By increasing or decreasing the inclination, the angle between the wellbore trajectory and the formation attitude is eliminated, and the trajectory of the horizontal section is controlled to travel through the dolomite of the bioherm body in the horizontal well being drilled.

[0044] Furthermore, in step 63c), the method for optimizing and adjusting the drilling trajectory is as follows: the trajectory tangent is determined based on the modified bioherm model. If the trajectory runs parallel, the current well inclination is maintained during drilling. If the trajectory is in a downward cutting posture, the trajectory is adjusted by increasing the inclination. If the trajectory is in an upward cutting posture, the drilling trajectory is optimized and adjusted by decreasing the inclination. According to the drilling engineering design requirements for horizontal wells, the inclination increase and decrease rates within the bioherm do not exceed 3° / 30m.

[0045] Furthermore, it also includes evaluation of the guiding effect during drilling.

[0046] Based on the trajectory map of the horizontal section of the bioherm body generated after the completion of drilling in step 6), the drilling rate is calculated by statistically analyzing the total length of the bioherm body encountered in the horizontal section and the actual length of the drilled horizontal section. The geological guidance effect is evaluated by the drilling rate.

[0047] A wellbore trajectory with a horizontal section encounter rate of ≥90% within a bioherm body, and a total angle variation rate of ≤3.0° / 30m at three consecutive measuring points, is considered excellent; a wellbore trajectory with a horizontal section encounter rate of <90% but ≥60% within a bioherm body, and a total angle variation rate of ≤3.0° / 30m at three consecutive measuring points, is considered moderate; a wellbore trajectory with a horizontal section encounter rate of <60% within a bioherm body, and a total angle variation rate of ≤3.0° / 30m at three consecutive measuring points, is considered poor.

[0048] The beneficial effects of this invention are:

[0049] 1) This invention provides a horizontal well geological steering method based on marker point + drilling natural gamma + gas logging + elemental logging + cuttings thin section + well deviation combined positioning and tracking of bioherms, which solves the problem of low drilling encounter rate of bioherms.

[0050] 2) This invention establishes a trajectory control method before drilling encounters a bioherm, forming a trajectory adjustment mode for the target entry stage based on the principle of "controlling well inclination, drilling vertical depth, exploring the Changxing Formation, and finding dolomite", which reduces the risk of missing the bioherm above due to excessive well inclination or missing the bioherm below due to insufficient well inclination.

[0051] 3) The successful application of this invention in the "one well, two reefs / shoals" configuration of the XL1-2 well in the Changxing Formation bioherm gas reservoir of the Xinglong Gas Field demonstrates that this technology has a wide range of applications and strong operability. The well completion test yielded a high-yield industrial gas flow of 801,300 cubic meters per day, breaking the record for the highest tested production of conventional natural gas in the Jianghan Oilfield. Attached Figure Description

[0052] Figure 1 This is a flowchart of the workflow of the present invention;

[0053] Figure 2 Map showing the delineation of marker points above the bioherm shoal in the Changxing Formation of the standard well;

[0054] Figure 3 A sub-layered diagram of the Changxing Formation bioherm shoal body in well XL1 (standard well);

[0055] Figure 4 An initial model of the distribution of Changxing Formation bioherm bodies along the trajectory of the horizontal section of the drilled horizontal well XL1-2 was designed.

[0056] Figure 5 This is the well inclination control chart corresponding to the marker point above the bioherm body of the Changxing Formation in the standard well XL1;

[0057] Figure 6 A vertical depth comparison diagram of the Feixianguan Formation above the Changxing Formation bioherm body in the horizontal drilling well XL1-2 and the standard well XL1;

[0058] Figure 7A comprehensive diagram showing the guidance of the bioherm body in the Changxing Formation of the horizontal well XL1-2 during drilling;

[0059] Figure 8 Photographs of thin sections of dolomite from the Changxing Formation in the horizontal well XL1-2, which is currently being drilled.

[0060] Figure 9 This is a trajectory diagram of the horizontal section of the Changxing Formation bioherm body in the XL1-2 horizontal well being drilled. Detailed Implementation

[0061] The following reference Figure 1 The present invention will be further described in detail with reference to specific embodiments to facilitate a clearer understanding of the present invention, but these descriptions do not constitute a limitation on the present invention.

[0062] The invention is described in detail using the directional drilling example of the XL1-2 horizontal well as an example:

[0063] Step 1: Collect geological data of drilled wells in the well area

[0064] Collect geological data from drilled wells in the well area, including formation lithology, drilling time, total hydrocarbons, methane, natural gamma, elemental logging data, and thin section data.

[0065] Step 2: Select the vertical well that encounters the bioherm as the standard well.

[0066] 1) Based on the drilling data in the well area, the vertical well XL1, which is closest to the bioherm encountered by well XL1-2, was selected as the standard well.

[0067] 2) The lithology of the bioherm body of Changxing Formation in Well XL1 is mainly medium to fine-grained dissolution-porous dolomite, with the main mineral components being magnesium carbonate and calcium carbonate. The rock cuttings showed a weak reaction to 5% hydrochloric acid, but turned blue when magnesium reagent was applied. Microscopic observation of thin sections showed a high content of bioclastics, but the alizarin red solution did not change color. The natural gamma value was low. The total hydrocarbon and methane values ​​in gas logging were significantly increased, and the drilling time was reduced. The magnesium content in elemental logging was significantly increased, while the calcium content was decreased.

[0068] Step 3: Selecting abrupt changes in the natural gamma curve morphology of standard wells as marker points. 1) Analyze the lithological and natural gamma characteristics of the formation within a 400.00m vertical thickness above the Changxing Formation bioherm body in well XL1. Focusing primarily on natural gamma curve characteristics and secondarily on lithological characteristics, select seven abrupt changes in gamma curve morphology within the Feixianguan Formation as target control marker points for well XL1-2. The marker points above the Changxing Formation bioherm body are named B1, B2, B3, B4, B5, B6, and B7 from top to bottom (see...). Figure 2 ).

[0069] Marker point B1: Boundary between the Jialingjiang Formation and the Feixianguan Formation, with a sharp peak in the natural gamma curve; the lithology ends with limestone, followed by purplish-red mudstone.

[0070] Marker point B2: Boundary between the third and second sections of the Feixianguan Formation, with a sharp-peaked median in the natural gamma curve; the lithology ends with gray limestone, followed by gray dolomite.

[0071] Marker point B3: In the middle of the second section of the Flying Formation, the natural gamma curve value rises overall, marking the end of the oolitic limestone formation.

[0072] Marker point B4: The boundary between the second and first segments of the flight curve, where the natural gamma curve shows a downward trend.

[0073] Marker point B5: In the middle of the flight segment, the natural gamma curve rises, showing two sharp peaks in the median.

[0074] Marker point B6: Flying through the lower part, the natural gamma curve shows three sharp peaks in the median.

[0075] Marker point B7: Boundary between the Feixianguan Formation and the Changxing Formation, with a natural gamma curve showing a sharp peak.

[0076] Step 4: Collect design and geophysical data for horizontal well drilling.

[0077] Collect drilling geological design and drilling engineering design for well XL1-2.

[0078] Collect geological structure map, seismic profile along the wellbore trajectory, wellbore trajectory design data, and target data for well XL1-2.

[0079] Step 5: Determine the stratigraphic position of the bioherm development in the horizontal well being drilled.

[0080] The bioherms and shoals of the Changxing Formation in the Xinglong Gas Field are mainly developed in the fourth sub-member of the second member of the Changxing Formation. The overall thickness of the Changxing Formation ranges from 280.00m to 320.00m. Based on variations in lithology, electrical properties, and dolomite content, the Changxing Formation is divided into two members. The second member is further subdivided into four sub-members, and the fourth sub-member of the second member is divided into five smaller layers (see...). Figure 3 The thickness ranges from 15.00m to 45.00m. The bioherm body in well XL1-2 corresponds to the fine-grained dissolution-porous dolomite section in the lower part of the fourth sub-section of the second section of the Changxing Formation in well XL1.

[0081] Step 6: Establish an initial model of the bioherm distribution along the trajectory of the horizontal section of the horizontal well during drilling.

[0082] Based on the drilling geological design, drilling engineering design, geophysical data, and bioherm development strata of well XL1-2 collected in steps four and five, an initial model of the Changxing Formation bioherm distribution along the designed horizontal trajectory of well XL1-2 was established using geological steering software (see...). Figure 4 ), predicting the thickness and depth of the bioherm along the trajectory of the designed horizontal segment.

[0083] Step 7: Calculate the controlled well inclination corresponding to each marker point above the bioherm body based on the standard well marker points and the designed wellbore trajectory data.

[0084] Based on the vertical depth of each marker point, the vertical thickness between each marker point, and the vertical thickness from each marker point to the designed target point A, combined with the designed wellbore trajectory and the designed entry angle, starting from the designed vertical depth of the Changxing Formation bioherm, the well inclination control corresponding to each marker point is calculated sequentially from bottom to top, establishing a segmented well inclination control chart corresponding to the marker points above the bioherm (see...). Figure 5 ).

[0085] Step 8: While drilling, create a formation comparison map between the horizontal well and the standard well to locate the drill bit position.

[0086] 1) Real-time collection of lithology, gas logging, natural gamma, elemental logging, and wellbore trajectory data for well XL1-2.

[0087] 2) The collected data were processed using geological mapping software to verticalize well XL1-2, and a vertical depth comparison map of the Feixianguan Formation above the Changxing Formation bioherm body in well XL1-2 and well XL1 was drawn (see...). Figure 6 ).

[0088] 3) Locate the current drill bit position based on the marker point above the bioherm body determined by comparing well XL1-2 with well XL1.

[0089] Step 9: Drilling while tracking the control trajectory to reach and penetrate the bioherm body.

[0090] 1) Track control before target entry is achieved by locating and tracking the well inclination segment control chart corresponding to the marker point.

[0091] ①Based on the vertical depth reached by each marker point encountered in well XL1-2 and the formation thickness of the corresponding standard well XL1, the vertical depth of the top of the Changxing Formation directly above target point A of the bioherm and the vertical depth of target point A are calculated point by point using the equal thickness method.

[0092] ② Based on the established wellbore inclination control chart corresponding to the marker points above the bioherm, and combined with the wellbore inclination of the actual drilling trajectory of well XL1-2, compare the actual wellbore inclination when reaching each marker point with the wellbore inclination required by the marker point wellbore inclination control chart. If the wellbore inclination error is within -2.0° to 2.0°, continue drilling according to the designed trajectory; if the wellbore inclination error is outside -2.0° to 2.0°, the build-up rate needs to be adjusted and the trajectory redesigned so that the wellbore inclination when reaching the next marker point is consistent with the wellbore inclination required by the marker point wellbore inclination control chart.

[0093] The formula for calculating the slope rate is:

[0094]

[0095] In the formula, K—slope rate, ° / m;

[0096] α—The difference in well inclination angle between two adjacent marker points, in degrees;

[0097] H—the vertical depth difference between two adjacent marker points, in meters.

[0098] Before reaching the target above the bioherm, well XL1-2 was drilled to B1 and B2. The well inclination was controlled according to the well inclination segment control chart corresponding to the marker points. The well reached a depth of 4395.00m and drilled to marker point B3. The actual drilled well inclination was 50.0°. According to the well inclination segment control chart for the marker point, the well inclination at this point should be 55.0°. The actual drilled well inclination was 5.0° smaller. The drilling trajectory needs to be adjusted so that the actual drilled well inclination when the trajectory reaches marker point B4 is consistent with the well inclination required by the well inclination segment control chart for the marker point. The vertical thickness between marker points B3 and B4 in standard well XL1 is 49.00m. The well inclination control chart for marker point segmentation requires an inclination of 63.0° to reach marker point B4, with a difference of 13.0° between the two points. Using the build-up rate calculation formula, the required build-up rate is 0.03° / m. In actual drilling, the well was increased at a build-up rate of 0.03° / m to a depth of 4538.0m, reaching marker point B4 with an inclination of 64.0°. After entering the first section of the Feixianguan Formation... The marker points were poorly identifiable. Drilling reached a depth of 4654.00m with an inclination of 74.85°. The stratigraphic position was the first section of the Feixianguan Formation. Drilling was carried out at an inclination of 74.0° to 75.0° to explore the top of the Changxing Formation. Drilling reached a depth of 4732.00m with an inclination of 74.8°. Marker point B7 was found at the gamma peak at the junction of the first section of the Feixianguan Formation and the Changxing Formation. The lithology changed from light gray limestone to gray dolomite-bearing limestone, gradually transitioning from limestone to dolomite. It was determined that the drilling had entered the Changxing Formation.

[0099] 2) Determine if a bioherm body has been reached based on the lithology, gas logging results, drilling time, and elemental logging characteristics of the standard well. Drill to the top of the Changxing Formation at an angle of 10.0°–15.0°. If no bioherm body is found after reaching the Changxing Formation, gradually increase the inclination to approximately 85.0°, then probe for bioherm bodies at an angle of approximately 5.0°. If a bioherm body is reached, the inclination can be increased and the trajectory adjusted.

[0100] XL1-2 well was drilled to 4774.00m with an inclination of 80.3°. The formation was the fourth sub-section of the second member of the Changxing Formation, with a lithology of gray dolomite. The natural gamma ray decreased from 25 API to 18 API, and the lithology changed from light gray limestone to dolomite. The rock cuttings showed a weak reaction to 5% hydrochloric acid but turned blue when treated with magnesium reagent. Microscopic observation of thin sections showed a high content of bioclastics and no color change when treated with alizarin red. Elemental logging showed that Mg increased from 0.9% to 8.7% and calcium decreased from 34.3% to 20.7%. Gas logging showed that the total hydrocarbon value increased from 0.66% to 24.32%. Based on the comprehensive judgment, the well reached a bioherm body. Drilling was then carried out at an inclination rate of 0.17° / 30m to 90.0°, and the target point A was reached at a depth of 4790.00m.

[0101] 3) By comprehensively utilizing lithology, gas logging, drilling time, elemental logging, cuttings thin sections, and natural gamma data while drilling, the bioherm distribution model is dynamically corrected on the initial model of bioherm distribution in the horizontal section trajectory direction of the horizontal well being drilled, and the drilling trajectory is optimized and adjusted to ensure the encounter rate of bioherms in the horizontal section.

[0102] ① Using data collected during drilling, a comprehensive guide map of the bioherm shoal in the Changxing Formation of Well XL1-2 was drawn (see...) Figure 7 The directional composite chart includes sub-layers, lithology, total hydrocarbons, methane, natural gamma ray during drilling, and elemental logging of aluminum, silicon, potassium, sodium, calcium, magnesium, barium, and strontium, totaling eight curves.

[0103] The biogenic reefs and shoals of the Changxing Formation are characterized on the navigation map by: low natural gamma values; high total hydrocarbon and methane values; and elemental curve characteristics: low aluminum and low silicon, low potassium and high sodium, high magnesium and low calcium, and a decreased barium-strontium crossover area. The abiotic reefs and shoals are characterized on the navigation map by: relatively high natural gamma values; low total hydrocarbon and methane values; and elemental curve characteristics: high aluminum and high silicon, low potassium and low sodium, high calcium and low magnesium, and an increased barium-strontium crossover area.

[0104] Through rock thin sections (see) Figure 8 Microscopic observation provides a direct understanding of the development of pores and fractures in the dolomite particles of the Changxing Formation bioherm body, and reservoir properties are evaluated during drilling.

[0105] ② Based on the real-time tracking and positioning of lithology, total hydrocarbons, natural gamma ray during drilling, and elemental logging data on the directional comprehensive map, the initial model of the bioherm body distribution in the designed horizontal section trajectory direction of the horizontal well during drilling is dynamically corrected. A trajectory traversal map of the Changxing Formation bioherm body in the horizontal well during drilling is generated. The trajectory is optimized and adjusted during drilling by increasing and decreasing the inclination, and the horizontal section trajectory of well XL1-2 is controlled to traverse within the dolomite of the bioherm body.

[0106] Based on geophysical data and actual drilling data, the designed trajectory of the bioherm body at target point A in well XL1-2 is estimated to be approximately upward dipping by about 1.0°. Furthermore, the formation between the first and second bioherm body sections dips upward by about 3.7°. Therefore, starting from a depth of 4889.00m, the drilling was initiated at a build-up rate of 0.06° / m to 92.6°, and then controlled at a controlled angle of 92.0°–93.0°. After controlling the horizontal trajectory to traverse 237.00m within the dolomite of the bioherm body, at a depth of 5027.00m, the natural gamma ray increased from 17 API to 30 API. The lithology changed from gray dolomite to gray limestone. Elemental logging showed that Mg decreased from 9.6% to 0.8%, calcium increased from 22.6% to 33.9%, and total hydrocarbons decreased from 11.56% to 0.99%. Based on these findings, it was determined that the first bioherm body section had ended. Drilling at a controlled angle of 92.5–93.5° to a depth of 5258.00m, the natural gamma ray decreased from 31 API to 18 API, the lithology changed from gray limestone to light gray dolomite, the elemental logging showed that Mg increased from 0.9% to 7.2% and calcium decreased from 35.0% to 27.1%, and the total hydrocarbon value in gas logging increased from 0.92% to 22.5%. Based on these findings, it was determined that the second section of the bioherm reservoir was encountered. Combined with geophysical exploration and actual drilling verification, the second section of the bioherm reservoir is distributed with a downward dip of about 0.7°. Starting at a depth of 5297.00m, the inclination was reduced to 89.0° at a build-up rate of 0.05° / m, and then drilling continued at a stable inclination. At a depth of 5656.00m, the natural gamma ray increased from 22 API to 46 API. The lithology changed from light gray dolomite to gray limestone. Elemental logging showed that Mg decreased from 10.2% to 1.3%, and calcium increased from 23.5% to 33.2%. Gas logging showed that the total hydrocarbon value decreased from 19.70% to 2.19%. Based on these findings, it was determined that the second bioherm body had ended.

[0107] Step 10: Evaluation of Guiding Effect During Drilling

[0108] Based on the horizontal trajectory map of the Changxing Formation bioherm body in well XL1-2 generated after the completion of the ninth drilling step (see...), Figure 9 The drilling rate is calculated by combining the total length of the horizontal section of the bioherm encountered with the actual length of the horizontal section drilled, and the geological guidance effect is evaluated by the drilling rate.

[0109] A wellbore trajectory with a horizontal section encounter rate of ≥90% within a bioherm body, and a total angle variation rate of ≤3.0° / 30m at three consecutive measuring points, is considered excellent; a wellbore trajectory with a horizontal section encounter rate of <90% but ≥60% within a bioherm body, and a total angle variation rate of ≤3.0° / 30m at three consecutive measuring points, is considered moderate; a wellbore trajectory with a horizontal section encounter rate of <60% within a bioherm body, and a total angle variation rate of ≤3.0° / 30m at three consecutive measuring points, is considered poor.

[0110] The XL1-2 well was designed with a vertical depth of 4375.00m for target point A and a horizontal section length of 711.00m, constituting a "one well, two reef-shoal bodies" configuration, meaning the horizontal section was designed to encounter two reef-shoal reservoir sections. The actual drilling depth at target point A was 13.00m deeper than designed. Using the geological steering control trajectory during drilling, the biogenic reef-shoal body was accurately encountered at the designed target point A coordinates. The drilling rate of both reef-shoal reservoir sections in the horizontal section was 100%, and the steering effect was rated as excellent.

Claims

1. A geological steering method for horizontal wells in bioherm gas reservoirs, characterized in that: The method specifically includes the following steps: 1) Collect geological data of drilled wells in the well area, as well as design data and geophysical data of horizontal wells being drilled; The design data for drilling horizontal wells includes drilling geological design and drilling engineering design. The drilling engineering design includes wellbore trajectory design data and target point data. Geophysical data for horizontal wells being drilled includes geological structure maps and seismic profiles along the wellbore trajectory. 2) Based on the geological data of existing wells in the well area, select the vertical wells that encounter bioherm bodies as standard wells; Based on the data of bioherm bodies encountered in the well area and the geological design of the horizontal well being drilled, the development strata of the bioherm bodies in the horizontal well being drilled were determined. 3) Select abrupt change points in the natural gamma curve morphology of standard wells as marker points; Based on the collected geophysical data of the horizontal well and the development strata of the bioherm bodies in the horizontal well, an initial model of the distribution of bioherm bodies along the trajectory of the designed horizontal section of the horizontal well was established. 4) Calculate the well inclination segment control chart corresponding to each marker point above the bioherm body of the standard well and the wellbore trajectory design data of the horizontal well being drilled. 5) While drilling, create a formation comparison map between the horizontal well being drilled and the standard well to locate the current drill bit position; 6) Drilling while tracking the control trajectory to reach and penetrate the bioherm body in the horizontal well being drilled; The specific process of step 6) is as follows: 61) Trajectory control before target entry is achieved by locating and tracking the well inclination segment control chart corresponding to the marker point; 62) Determine whether the bioherm body has been reached based on the lithology, gas logging, drilling time, and elemental logging characteristics of the standard well bioherm body; drill at an angle of 10.0° to 15.0° to reach the marker point closest to the designed target point A of the horizontal well; if no bioherm body is found after reaching the designed target point A of the horizontal well, gradually increase the inclination to 83.0° to 87.0°, and probe for the bioherm body at an angle of 4.0° to 6.0°; if the bioherm body is reached, adjust the inclination and level the trajectory. 63) By comprehensively utilizing lithology, gas logging, drilling time, elemental logging, cuttings thin sections, and natural gamma data while drilling, the bioherm distribution model is dynamically corrected on the initial model of bioherm distribution in the horizontal section trajectory direction of the horizontal well being drilled, and the drilling trajectory is optimized and adjusted to ensure the encounter rate of bioherms in the horizontal section.

2. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: In step 1), the geological data of the drilled wells in the well area include formation lithology, drilling time, total hydrocarbons, methane, natural gamma, elemental logging data, and rock thin section data.

3. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: In step 2), based on the geological data of the wells already drilled in the well area, the vertical well that is closest to the horizontal well being drilled and has encountered a bioherm is selected as the standard well.

4. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: In step 3), based on the formation lithology and natural gamma characteristics of the standard well determined in step 2), the abrupt change in the natural gamma curve morphology is selected as the target control marker point for drilling horizontal wells.

5. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: In step 3), based on the collected geophysical data of the horizontal well and the development strata of the bioherm body in the horizontal well, the geological guidance software is used to establish an initial model of the lateral and longitudinal distribution of the bioherm body in the horizontal section trajectory direction of the horizontal well. The size and lateral and longitudinal distribution characteristics of the bioherm body in the horizontal well are characterized, and the thickness and depth of the bioherm body in the designed horizontal section trajectory direction are predicted.

6. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: In step 4), based on the vertical depth of each marker point, the vertical thickness between each marker point, and the vertical thickness from each marker point to the designed target point A, combined with the wellbore trajectory design data and the designed entry angle, the well inclination corresponding to the control point is calculated from bottom to top, starting from the vertical depth of the bioreef body in the horizontal well being drilled, and the well inclination segment control chart corresponding to the marker point above the bioreef body in the horizontal well being drilled is established.

7. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: The specific process of step 5) is as follows: 51) Real-time collection of lithology, gas logging, natural gamma ray, elemental logging, and wellbore trajectory data for horizontal wells being drilled; 52) The collected data will be processed into a vertical well using geological drawing software, and a vertical depth comparison chart of the formation between the drilled horizontal well and the standard well will be drawn. 53) Locate the current drill bit position based on the marker point above the bioherm body of the horizontal well determined by comparing it with the standard well.

8. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: The specific process of step 61) is as follows: 61a) Based on the vertical depth reached when each marker point is encountered in the horizontal well and the corresponding formation thickness of the standard well, the vertical depth of the top of the bioherm body designed for target point A and the vertical depth of target point A are calculated point by point using the equal thickness method. 62b) Based on the well inclination control chart corresponding to the marker point above the bioherm body established in step 4), and combined with the well inclination of the actual drilling trajectory of the horizontal well, compare the actual well inclination when drilling to each marker point with the well inclination required by the well inclination control chart of the marker point; if the well inclination error between the two is within -2.0° to 2.0°, continue drilling according to the designed trajectory; if the well inclination error between the two is outside -2.0° to 2.0°, adjust the build-up rate and redesign the trajectory so that the well inclination when drilling to the next marker point is consistent with the well inclination required by the well inclination control chart of the marker point. The formula for calculating the slope rate is: In the formula, K—slope rate, ° / m; α—The difference in well inclination angle between two adjacent marker points, in degrees; H—the vertical depth difference between two adjacent marker points, in meters.

9. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: The specific process of the optimization and adjustment is as follows: 63a) Use data collected while drilling to draw a directional composite chart. The directional composite chart includes sub-layer, lithology, total hydrocarbon, methane, and natural gamma ray while drilling. Elemental logging selects eight curves for aluminum, silicon, potassium, sodium, calcium, magnesium, barium, and strontium. 63b) Based on the lithology, total hydrocarbons, natural gamma ray during drilling, and elemental logging data on the directional composite chart, combined with cuttings thin section and well inclination data for real-time tracking and positioning, the initial model of bioherm distribution along the designed horizontal section trajectory of the horizontal well is dynamically corrected, and a bioherm horizontal section trajectory map is generated during drilling; Model adjustment method: Calculate the difference between the vertical depth of the bottom interface of the bioherm at the actual drilling location and the vertical depth of the bottom interface of the initial model at the corresponding closure displacement. If the difference is positive, the initial model needs to be moved vertically downward; if the difference is negative, the initial model needs to be moved vertically upward. The magnitude of the model movement is consistent with the difference. A difference of 0 indicates that the actual drilling results match the initial model, and the initial model does not need to be adjusted. 63c) By increasing or decreasing the inclination, the angle between the wellbore trajectory and the formation attitude is eliminated, and the trajectory of the horizontal section is controlled to travel through the dolomite of the bioherm body in the horizontal well being drilled.

10. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 9, characterized in that: In step 63c), the method for optimizing and adjusting the drilling trajectory is as follows: the trajectory tangent is determined based on the modified bioherm body model. If the trajectory runs parallel, the current well inclination is maintained. If the trajectory is in a downward cutting posture, the trajectory is adjusted by increasing the inclination. If the trajectory is in an upward cutting posture, the drilling trajectory is optimized and adjusted by decreasing the inclination. According to the drilling engineering design requirements for horizontal wells, the inclination increase and decrease rates within the bioherm body shall not exceed 3° / 30m.

11. The geological steering method for horizontal wells in bioherm gas reservoirs according to claim 1, characterized in that: It also includes the evaluation of the guiding effect during drilling. Based on the trajectory map of the horizontal section of the bioherm body generated after the completion of drilling in step 6), the drilling rate is calculated by statistically analyzing the total length of the bioherm body encountered in the horizontal section and the actual length of the drilled horizontal section. The geological guidance effect is evaluated by the drilling rate.

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