The study area is part of a producing field where the reservoir pressure has been depleted to 3450 psi from virgin reservoir pressure of 5235psi at top of gas reservoir due to production. This paper represents how the integrated reservoir geomechanics was used for efficient well planning and safe drilling of two very first nearly horizontal wells (78° - 82° Inclination) through this depleted sandstone gas reservoir of Cretaceous age in offshore deep water (1175m TVDSS) Krishna-Godavari basin, East Coast of India.
Stress changes due to pressure depletion in reservoir causes horizontal stresses to decrease, whereas shear failure gradient (minimum mud weight requirement to prevent borehole collapse or breakouts) may still remain higher depending on stress magnitudes, strength of sandstone reservoir, intra-reservoir shale and wellbore trajectory. Thus, drilling a very high angle well through depleted reservoir in an offshore deepwater condition generates big challenges compare to drilling in pre-production stage of field. An integrated reservoir geomechanics methodology were used to minimized the uncertainties of different geomechanical parameters in order to accurate estimation of shear failure gradient and horizontal stress reductions. The Geomechanical model built by integrating rock mechanical (Lab test data), geological (3-D Reservoir model), petrophysical (raw logs, processed logs), reservoir (pressure behaviour), drilling data and production data has lowered the uncertainty in model and reduced risks associated for drilling.
This integrated reservoir geomechanics approach results in optimum drilling plan mud weight window in the depleted (depleted by ~ 1800psi) reservoir section for the planned well (78° Inclination) and well was drilled successfully without any drilling complications/events. After verifying the applicability of the Mechanical Earth Model, another near-horizontal well (82° Inclination) through depleted reservoir of this field was drilled successfully. Methodology adopted in this paper also highlights how a reliable geomechanical model can be built for a field which having geomechanical parameters related data constraint. Approach adopted in this paper can be used for well planning and drilling of future virgin as well as depleted vertical or deviated wells in East Coast of India.
Paper illustrates the advantages of using 3-D Reservoir model based seismic attributes (P-Impedance) derived porosity and facies data for more reliable estimation of rock strength in the reservoir rocks encountered along the well path rather than directly using offset well porosity data. MCC (Mechanical Consolidation Curve) for clean sandstone (Porosity > 0.2) has been generated for East Coast of India to reduce uncertainty on rock strength estimation. This MCC will be an effective alternative tool to predict reliable rock strength values in any reservoir section of Krishna-Godavari basin.