With the increase in deepwater exploration and unconventional tight gas exploration, wellbore stability analysis has emerged as a pivotal part of finalizing drilling programs. It becomes more important when inclined wellbores are drilled where stable mud weight windows are narrow. Wellbore stability analysis is conducted to validate calculated stress and rock properties against drilling observations. Uniaxial compressive rock strength (UCS) is an integral part of geomechanics studies involving mechanical earth modeling. Absence of direct core measurement brings uncertainty in the model. Presently, the industry uses different empirical relationships for estimation of UCS depending on formation type. It is critical to use appropriate relationship with minimum error and known quantification effects on final mud weight recommendation. Depositional sequence brings formation under different compaction regime of grains with interstitial pore fluid. Sandstone and shale are two common lithology found in India deepwater exploration field with intermediate siltstone. Validation of different models depends on porosity, volume of clay, saturation and sonic slowness response of formation. The present paper reviews these methods with four case studies from India and discusses how different models respond in deepwater formation sequences. Best method depending on range of volume of clay and porosity has been discussed in term of fuzzy logic. Mathematical model is build for uncertainty quantification of effect on mud weight optimization using different methods for UCS estimation is also discussed.

A detailed, geotechnical study and numerical modelling were~one for optimum slope design of a deep (170 m) open pit mine in Rajasthan state of India. The study includes physical characterization of the discontinuities in and around the mine, geomechanical propertis of the slope materials, estimation of Rock Mass Rating and rock mass properties. The data related to different geotechnical parameters were collected from the benches of the partially developed mine and logs of the boreholes. Further, numerical modelling was done for slope stability and sensitivity analyses, based on which optimum slope angle was determined. The study revealed 42-and 48-overall footwall and hangingwall slope angles respec·tively. Sensitivity analysis shows that ground water is detrimental to the slopes. Hence, every attempt should be done to control the ground water condition.

As the exploration for hydrocarbon resources continues to move into nontraditional areas, geologists are targeting reservoir rocks not usually associated with typical sedimentary-basin settings. One such group of rocks are volcanic rocks. In drilling 10 wells in the Deccan volcanic province in the Ingoli field in the Cambay basin, all wells encountered thick sections of Deccan basalt with intermittent fine-grained intratrappeans. However, only three of the wells produced hydrocarbon. In this formation, conventional resistivity was unable to distinguish fluid saturations, so defining the hydrocarbon-bearing zones was not possible. A nonconventional and integrated approach successfully characterized the formation and the reservoir. Using image logs, together with mud logs, we defined the facies that could be productive in this basalt formation. All the geological information was used in material balance analysis to estimate possible ranges of original oil in place (OOIP). Well data, particularly borehole images, seismic attribute interpretation, geomechanical analysis, and reservoir and production data were used to understand the reservoir system, characterize the facies, and provide guidelines to delineate the reservoir. Significant uncertainties had to be addressed to ensure successful exploitation of significant remaining oil in place. A number of multibean productivity tests and pressure buildup tests were acquired. Pressure transient analysis of these well test data, when incorporated with borehole image interpretation, provided important insights in understanding the productive reservoir facies. A conceptual geological model for the Ingoli basement reservoir is developed by integrating all the boreholederived geological information with seismic and reservoir analysis. The conceptual geological model forms a basis to consider the next steps of effectively exploiting the Ingoli field. Introduction Exploration and exploitation of hydrocarbon from unconventional reservoirs pose new challenges to subsurface geologists where conventional technology and proceses may not all be applicable. Worldwide basement rocks have indicated significant oil resources, but only a small percentage (Byeonggoo Choi, et. al., 2008) have been exploited. The Deccan basement of the Ingoli field of the Cambay basin has been identified as hydrocarbon bearing volcanic reservoir. Gujarat State Petroleum Corporation (GSPC) drilled ten wells through the Deccan volcanic in the period 2003-2004 in the north-south trending ridge, called the PK high (See details in Geophysics Section), in the Cambay basin (Fig. 1). All the wells encountered thick siliciclastic sequence underlying Deccan basalt with intermittent fine-grained intratrappeans. The prime target here is fractured and weathered basaltic rock. The general stratigraphy (Fig. 2) outlines the source, reservoir, and caprock present in this part of the basin. The uplifted older basement block proved to be a potential reservoir against younger Cambay shale, which is a major source rock. The structural framework in this region is characterized by three fault trends, namely, Dharwarian, the oldest, trending north-northwest to south-southeast; Aravalli, trending northeast to southwest; and Satpura, the youngest, trending east-northeast to west-southwest. These trends define the fault system of the study area. The Dharwarian fault system is believed to be the conduit for hydrocarbon migration from source to the reservoir.

Assurance of wellbore stability (WBS) is of utmost concern and a key challenge in drilling an inclined well in ultra deep water of the East Coast of India. The WBS analysis requires accurate modeling of earth stresses and rock mechanical properties. These processes are primarily based on sonic logs (compressional and shear slowness), bulk density and lithological distribution. To understand and address drilling complications in the study area, post-drill (offset well analysis) and real-time drilling geomechanics is carried out in this well. 1D mechanical earth model (MEM) and WBS model is constructed for offset wells, which is calibrated with caliper log, pressure test and leak-off data sets. WBS analysis suggested drilling with lower mud weight in the zones of shear failure and pack-off. Disparity in resistivity values is also observed when wireline logs and Logging-While-Drilling (LWD) logs are analyzed. This might be due to mud invasion or fluid-shale interaction in the open hole, as it is resolved by changing mud system from water based mud (WBM) to synthetic oil base mud (SOBM). The post-drill analysis of offsets wells established parameters for the upcoming inclined well. The planned well is the first inclined well (horizontal drift more than ∼2000m) in ultra deepwater of the East Coast of India to avoid the drilling risks; real-time drilling geomechanics is first time put into operation. Required sonic and density data is received in reasonable time intervals to perform real-time analysis. Timely updates on rock mechanical properties are provided to client, which helped in optimizing drilling parameters. As a result, first inclined well in ultra deep water in the East Coast of India was drilled successfully.

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