Matrix treatment of damaged sandstone formations using buffered HF-acidizing solutions

Abstract

Sandstone formations of oil and gas and geothermal wells are effectively stimulated when a buffered HF-sandstone acidizing solution is employed without the prior introduction of an acid containing preflush solution. By not using a preflush solution, buffered HF-sandstone acidizing solutions are highly effective in dissolving and removing siliceous material while minimizing the formation of calcium fluoride.

Description

[0001] This application claims the benefit of U.S. Patent Application Ser. No. 60 / 637,134, filed on Dec. 17, 2004.FIELD OF THE INVENTION [0002] The invention relates to a method of stimulating or remediating sandstone formations of oil and gas and geothermal wells without the use of preflush acidizing solutions. BACKGROUND OF THE INVENTION [0003] In the course of drilling, or during production or workover, the vast majority of oil and gas wells are exposed to conditions that ultimately lead to formation damage. Formation damage limits the productive (or injective) capacity of the well. The reduction in well performance is generally due to changes in near-wellbore permeability which may be caused by a number of factors, such as rock crushing, invasion of drill solids, swelling of pore-lining clays, migration of mobile fines and changes in wettability. [0004] It is known that permeability impairment may be improved by injecting acid formulations containing HF into the formation. Such ...

AI Technical Summary

Benefits of technology

[0033] Thus, the invention minimizes the risk of iron formation and further minimizes the need for use of rust dissolvers. By eliminating the use of a mineral acid preflush and using a buffered HF-acidizing solution in accordance with the invention, problems associated with iron dissolution and its subsequent precipitation are largely mitigated. Such an approach, when coupled with a tubing pickle, such as a neutral chelant pickling agent, significantly improves acidizing in many formations. A particular advantage of the invention is the ability to inject a neutral chelant pickling agent, containing the dissolved and complexed iron, etc., directly into the formation without having to reverse it out ahead of the acid treatment. Suitable neutral chelant pickling agents include conventional inert water-soluble polymeric chelants known in the art which are capable of chelating a polyvalent metal ion. These include polymeric chelants having a molecular weight of between about 600 and about 1,000,000.

[0034] In addition to its use in matrix acidizing, the invention is applicable in remediation of oil and gas and geothermal wells by the removal of unwanted deposits from the wellbore and production equipment. Such unwanted deposits form and / or accumulate in the wellbore, production and recovery equipment and well casing. Such accumulated deposits affect productivity and are typically removed prior to cementing or the introduction of completion fluids into the wellbore. Remediation treatment fluids are further typically used to remove such undesired deposits prior to the introduction of stimulation fluids. In a preferred embodiment, the invention is used to remove siliceous deposits inside well tubulars.

[0035] In well remediation applications, the acidizing solution is preferably injected directly into the wellbore through the production tubing or through the use of coiled tubing or similar delivery mechanisms. Once downhole, the solution remedies damage caused during well treating such as, for instance, by stimulation fluids and drilling fluid muds, by dispersing and removing siliceous materials from the formation and wellbore. EXAMPLES

[0036] The following examples are illustrative and should not be construed as limiting the scope of the invention or claims thereof.

[0037] Unless otherwise indicated, all percentages are expressed in terms of weight percent.

[0038] BJ Sandstone Acid (BJSSA), a product of BJ Services Company, was employed as the buffered HF-acidizing solution.

Problems solved by technology

In the course of drilling, or during production or workover, the vast majority of oil and gas wells are exposed to conditions that ultimately lead to formation damage.

Formation damage limits the productive (or injective) capacity of the well.

Unfortunately, the process of dissolving siliceous minerals is not simple.

The generation timing of such secondary and / or tertiary precipitates as well as their placement in critical locations, such as the near wellbore, ultimately may cause further formation damage and negate the benefit of the acid treatment.

This strategy dilutes the concentration of reaction products per unit volume of acid treating solution but, unfortunately, also reduces the total quantity of siliceous minerals that can be dissolved per unit volume of acid.

Such systems essentially limit the availability of hydrogen ions for generation of HF.

Unfortunately, the secondary and tertiary precipitates generated by the interaction of HF with siliceous minerals are not the only problematic byproducts encountered when acid formulations containing HF enter a sandstone rock matrix.

Calcium fluoride precipitates quickly and, instead of stimulating the formation, causes formation damage by blockage.

While theoretically sound, such preflushes are not always as successful as desired.

Since treatments are very complex—involving many repeat stages of preflush, main HF-stage, overflush as well as diverter—it is often difficult to ensure that the acid stage is properly entering the desired zone and encountering the appropriate mineralogy.

This may result in very poor zonal coverage, poor damage removal, creation of unexpected damage due to acid / rock incompatibilities and, ultimately, poor stimulation results.

These problems are especially evident when the majority of commercial, HF-containing acid systems are employed.

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