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SPE/ISRM 78161

Abstract
In the petroleum industry, mud rocks and shales constitute more than 75% of the drilled formations and continue to be the most problematic and costly formations to drill[1, 2]. The cost of shale related drilling problems was estimated to be in the range from $400 to $500 million per year (excluding former communist countries)[2]. Recent advancements in the physico-chemical interaction between drilling muds and shales[3,4,6,7,15] suggest that evaluations of the relative magnitudes of water exchange (by hydraulic flow and osmotic flow) and ionic exchange between these two systems hold the key to alleviating shale wellbore stability problems. The scratch tester, because of its capacity for continuous, localized, surface measurements has the potential for providing these assessments in a rapid and cost effective manner.

The scratch test technique has been used to study the effects of exposing Pierre-1e shale samples to KCl brines of various concentrations for different lengths of time. The purpose of this study was to determine if this relatively new technique would prove useful as a screening tool for shale-mud compatibility evaluations. Scratch measurements were conducted by scratching the external surface of shale samples with a sharp polycrystalline diamond cutter at constant velocity and pre-determined depth of cut, while continuously measuring the horizontal and vertical forces on the cutter. Results from scratch testing provide the relative intrinsic specific energy required to remove a unit volume of rock. Hence, the physico-chemical mud-shale interaction can be determined from measurements of the intrinsic specific energy as a function of exposure time and depth from the surface of exposure. At small depths of cut, the intrinsic specific energy of the rock is directly related to its unconfined compression strength. Thus, as subsequent scratches with identical depth of cut are conducted over the same area, the depth and rate of penetration of the physico-chemical mud-shale interaction lead to local changes in rock strength that are measured directly as a function of depth

Fractures in shale samples, originating from core handling, stress relief, or desiccation, often tend to complicate the interpretation of fluid compatibility studies based on bulk rock behavior. The scratch test technique is site specific and the location of fractures and otherwise weakened zones within the tested sample are detected readily. Furthermore, the scratch test provides continuous, high resolution, measurements along the entire length of the sample and thus provides a quantitative measurement of its heterogeneity.

This study was conducted using Pierre-1e shale after immersion for different periods of time in non-reactive laboratory mineral spirit (OMS), and three different KCl brine solutions at 11%, 16% and 22% by weight. Short-term (2 to 5 hours) and long-term (18 to 22 hours) effects of KCl brine exposure were measured. Both conditions generally resulted in changes in rock strength; however, for short-term exposure to brines the results were inconclusive. Long-term exposure to brines resulted on marked reduction in rock strength. Diffusion constants calculated from the measured thickness of the diffusion layer and the time of exposure, suggest that (i) other mud-shale interactions faster than ionic diffusion may contribute to the loss of rock strength or that (ii) the rates of ionic diffusion are considerable faster under unstressed conditions.

Based on these results, we believe that the scratch tester is a viable methodology for efficient evaluation of mud-shale compatibility on core samples. Mud system developers and enginners involved in selecting drilling muds systems for shales should be the prime beneficiaries of the scratch test methodology.