Method and tool for evaluating a geological formation

Abstract

A method of evaluating a geological formation (20) of non-homogeneous porosity and permeability, particularly in exploration for hydrocarbon fluids, comprising the steps of:

• • (a) inserting a formation evaluation tool (30) into a borehole (10) to a location (10a) within the geological formation (20);
  • (b) setting the formation evaluation tool (30) into position isolating interval(s) of the borehole (10) with a packer arrangement (50) forming portion of the formation evaluation tool (30), for sample evaluation for said interval(s);
  • (c) extracting representative samples of formation fluid from the location within the geological formation (20) for evaluation under downhole conditions using sampling means (32a, 52b, 54b) operative to extract formation fluid samples over a range of porosity and permeability encountered within the geological formation (20); and
  • (d) analysing the formation fluid samples and measuring formation (20) permeability.

A formation evaluation tool (30) for practising the method, and including an packer arrangement (50) for isolating intervals of the borehole (10) for sample acquisition and evaluation, is also disclosed. The method and tool (30) are particularly suited to geological formations comprising complex carbonate rocks such as mudstones, Wackestone, Packstone, Boundstone and crystalline carbonate rocks which typically include features such as vugs and fractures.

Description

FIELD OF THE INVENTION[0001]This invention relates to a method and tool for evaluating a geological formation, particularly a formation containing hydrocarbon fluids.BACKGROUND TO THE INVENTION[0002]Oil and gas exploration involves investigation of geological formations for locating hydrocarbon reserves fit for commercial exploitation. Such geological formations may be termed reservoir formations. Oil and gas explorers are constantly trying to find better, more reliable, faster and cheaper ways to assess the potential of prospective reservoir formations as exploration costs can range into the tens and hundreds of millions of US dollars. This is a significant challenge as reservoir geology may be highly complex.[0003]To assist this process, the Applicant has developed a formation evaluation device or tool suitable for evaluating geological formations containing hydrocarbon reservoirs. Such geological formations tend to be porous and permeable, the pores containing hydrocarbon fluids ...

AI Technical Summary

Benefits of technology

[0040]The packer arrangement may conveniently be deployed in a straddle configuration to evaluate formation properties and especially vertical continuity of any reservoirs or reservoir compartments. Such deployment of packers is beneficial as testing either of the dual packers would permit sufficiently rapid acquisition of samples as described above. Further, under certain safe conditions, both packers may acquire pressure build up curves or pressure draw down curves to get shut in pressures (SIP) that may be used directly to measure hydraulic gradients from which fluid density and other fluid properties can be calculated. For example, analysis of pressure build up and draw down curves, and / or measuring the differential pressure between packers spaced a fixed distance apart, allows determination of the density of fluids, such as hydrocarbons, present at a location within the geological formation taking downhole conditions into account. Traditional gradient testing has depended upon cable displacements based upon surface measurements of such displacements with the erroneous assumption that surface displacements are the same as downhole displacements and this false assumption results in errors on the computed hydraulic gradient and inaccurate evaluation of the formation and potentially costly errors in assessing the value of hydrocarbon fluids in the formation. As described above, this is a particularly acute problem in the case of carbonate formations.

[0041]The method and tool of the invention allow convenient investigation of vertical continuity and other hydraulic properties within a geological formation. Vertical continuity is particularly important since nearly all reservoirs are compartmentalised, with low permeability structures or strata separating compartments containing hydrocarbon fluids and water, each compartment likely containing fluids with different properties. Such low permeability structures may prevent or restrict vertical flow, causing a barrier to commercial exploitation of a reservoir. Investigation of the vertical continuity of such compartments or reservoirs allows identification of vertical fluid flow barriers and so the likely performance of a reservoir, and its value, to be estimated.

[0042]For example, if one packer is drawn down and pressure at the location of the other packer monitored then an understanding of vertical continuity of the formation can be inferred. This allows better evaluation of hydraulic properties of fluids within the geological formation, particularly the permeability of the geological formation which dictates whether hydrocarbons can be economically extracted, and a more useful assessment of hydrocarbon reserves and value. In this way, the formation evaluation tool allows measurement of permeability of the geological formation with reference to conditions downhole and therefore in a more precise manner than done previously.

[0043]Identifying the representative nature of the formation fluid samples, once extracted by the sampling means, is an important feature of the method and tool. Identifying sample representativeness is based on monitoring a signal from a sensor arrangement desirably included within the formation evaluation tool The sensor arrangement includes at least one and preferably more of conductivity, resistivity, temperature, flow-rate, pressure and density sensors. Signals from the selected sensors are transmitted in real time to the surface where an operator or control unit determines representativeness of the formation fluid sample. The operator or a control unit may determine the best time and means to obtain a truly representative formation fluid sample. Consistency of sensor signals indicates sample representativeness and achieving such consistency may at least take significant and undesirable exploration time, with consequential costs, in many conventional exploration operations. The signals from the sensors are critical to correctly identifying components and characteristic properties (pressure, temperature, conductivity, density etc) of the formation fluid(s). Such sensors, and any additional sensors, enable evaluation of formation fluid samples under downhole conditions to avoid problems of sample contamination or variation in the properties of formation fluid samples when conveyed to surface where pressure and temperature conditions may be very different, providing erroneous analysis of the formation and hydrocarbon reserves present within the formation.

[0044]The method and tool allow for accurate evaluation of geological formations containing hydrocarbon resources even where such formations have a wide range of porosity and permeability such as in the case of carbonate or complex carbonate rock types. At the same time, the flexibility of the sampling means in coping with variation in porosity and permeability allows for faster and less costly exploration, this being a very important advantage in an industry where exploration efforts may cost in the tens or hundreds of millions of US dollars.

Problems solved by technology

This is a significant challenge as reservoir geology may be highly complex.

Carbonates are rigid, especially when well cemented, and thus readily fracture when folded.

Such rocks are thus often over pressured.

Evaluation of formations containing the complex carbonate rock types is challenged by the typical considerable variation or heterogeneity in rock porosity and permeability caused by the above phenomena.

Such variation may occur even over a very short interval, or within a relatively small region, within a geological formation causing, during formation evaluation, risk of over or under-estimating hydrocarbon reserves.

In addition, non-homogeneous and large pore size, fractures and vugs are likely to cause significant difficulties with sealing of conventional sample pads against borehole walls and this affects formation evaluation as well.

In fact, sealing may not be possible.

Such sample pad designs which depend on direct sealing between sample pad, having a sample port formed in the surface, and borehole wall are not effective for extracting formation fluid samples from formations including fractures and vugs.

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