Cementing fluid for enhancing the stability of a water sensitive, reactive subterranean formation

Abstract

The current invention provides a cementing fluid for enhancing the stability of a water sensitive reactive subterranean formation. The cementing fluid comprises a compound capable of adjusting the water activity of the cementing fluid. The preferred cementing fluid has a water activity lower than the pore fluid found in the formation.

Description

[0001] This Application is a Divisional of Ser. No. 101298,251 filed on Sep. 2, 2002BACKGROUND OF THE INVENTION [0002] The current invention relates to a method for enhancing formation stability during well construction. The method of the current invention improves the process of formulating cementing fluids (flushes, spacers, and cement slurries) such that the fluids reduce the risk of formation instability during well completion operations. More specifically, the current invention relates to a test for determining the optimum cementing fluid formulation for use in water sensitive, reactive formations. [0003] Water sensitive, reactive formations include but are not limited to marl, clay bearing sandstone, clay bearing carbonates, shale stringers in salt formations and carbonate formations. Shales are among the most commonly encountered formations. Shales are fine-grained sedimentary rocks composed of clay, silt and in some cases fine sands. For the purpose of this discussion, shale...

AI Technical Summary

Benefits of technology

[0011] Additionally, the current invention provides a method for enhancing the stability of a water sensitive, reactive formation penetrated by a wellbore during the cementing process. The method of the current invention comprises formulating cementing fluids having low water activity levels and capable of applying an osmotic semi-permeable membrane on the face of the formation. According to the method of the current invention, a sample of the targeted subterranean formation or formation of similar composition is obtained and placed under a confining pressure in a suitable testing device. Using a fluid similar to formation pore fluid (simulated pore fluid), back-pressure approximating the in situ pore pressure of the formation is applied to the sample. Thereafter, removing the back-pressure while maintaining the confining pressure consolidates the sample. Subsequently, upstream pressure is applied to the sample using the simulated pore fluid while monitoring the downstream pressure. Once the downstream pressure has increased by at least 50 percent, the upstream pressure is removed and the upstream and downstream pressures are allowed to approximately equilibrate. Following stabilization of the upstream and downstream pressures, upstream pressure is once again applied to the sample using a cementing fluid. During the application of upstream pressure, the change in downstream pressure is measured. In response to the change in downstream pressure, the water activity of the cementing fluid may be increased or decreased. Additionally, prior to or during the cementing operation, an osmotic semi-permeable membrane is applied to the face of the subterranean water sensitive, reactive formation.

Problems solved by technology

Argillaceous formations like shales make up over 75 percent of drilled formations and cause over 90 percent of wellbore instability problems.

Instability in shales is a continuing problem that results in substantial annual expenditure by the petroleum industry—in excess of a billion dollars according to conservative estimates.

However, the use of salts in cement slurries has not been consistent with respect to formation issues.

However, very little actual supporting evidence for this assumption has been found.

Although very meaningful to the understanding of that specific issue, any connection between effects on clays in permeable sandstones and formation instability as related to shales is complicated by precipitation of various calcium salt species from cement slurries.

When salts are applied, presumably for formation stability purposes, it is frequently done without a true understanding of the method or outcome.

Additionally, use of salts specifically in cementing spacers and preflushes is seldom applied.

However, combining salts and fluid loss additives in the same slurry frequently presents a more complicated and costly scenario because many fluid loss additives do not hydrate and / or otherwise function as efficiently in the presence of high concentrations of soluble salts.

This cost-driven approach to achieving cement slurry fluid loss values has resulted in the reduction and general elimination of salts in most primary cementing slurries without a true understanding of the resulting effects on wellbore stability.

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