How to Diagnose Drilling Induced Fractures in Wells Drilled with Oil-Based Muds with Real-Time Resistivity and Pressure Measurements

Abstract

Abstract Major problems are often encountered in relaxed basins when extended reach wells are drilled through depleted reservoirs. As wellbore inclination increases, the imbalance between vertical and horizontal stresses can cause formation breakouts leading to increased cuttings and increasing the potential for stuck pipe. Higher mud densities can stabilize the imbalance and facilitate cuttings transport, but increase the risk of differential sticking and lost circulation. Additionally, higher mud densities can create fractures that take mud while drilling and return mud during connections. This ‘ballooning’ or ‘weeping’ complicates the correct diagnosis and increases the risk of losing the well. Early identification of these competing mechanisms can be critical to successful drilling. Real-time resistivity-at-the-bit images are now possible to aid diagnosis, but are currently limited to water-based muds, and a limited range of conductive oil-based muds (OBMs). Nevertheless, conventional resistivity measurements can still be used in wells drilled with OBM. A case study is described of a highly deviated Gulf Coast well drilled with synthetic OBM that penetrated a severely depleted reservoir. Based on the data collected the original assumption that depleted sands were the only source of lost return zones was in error. The losses were found to be in the bounding shales as well. After losing two wellbores, the project was abandoned due to wellbore instabilities and limited reserves. Investigations into the lessons learnt highlight how multiple passes with both resistivity and annular pressure measurements could have been used to diagnose the location and mechanism of borehole failure, and hence suggest appropriate action. Indeed, the resistivity measurements were found to be responding to induced fractures hours before any changes in equivalent circulating density (ECD) or significant drilling observation. A methodology is given for diagnosing drilling induced fractures from the real-time measurements, so that remedial actions can be promptly taken. Success in future operations will come from including these new methods into the drilling plan. Introduction The use of resistivity images to distinguish between natural and drilling-induced fractures has been described by Rezmer-Cooper et al.1,2 While drilling, it is important to distinguish natural features from those induced by the drilling process so that the drilling program can be modified to minimize the impact of the induced fractures. A geological analysis of borehole images includes the search for open natural fractures. Wrongly identifying drilling-induced fractures as natural fractures results in an optimistic forecast, and could lead to incorrect remedial procedures being recommended for the drilling program. However, even though real-time images are now possible, and can now complement existing conventional real-time logging-while-drilling (LWD) measurements, their use is limited to water-base muds or conductive oil-base muds, which are still in their infancy, and have yet to gain wide acceptance.