Detection of overpressure zones and a statistical model for pore pressure estimation from well logs in the Krishna-Godavari Basin, India

Abstract

Abnormally high pressures, measured by repeat formation tester (RFT) and detected by well log data from 10 wells in the Krishna-Godavari (K-G) Basin, occur in the Vadaparru Shale of Miocene and Raghavapuram Shale of Early Cretaceous age. Overpressures generated by disequilibrium compaction, and pore pressures have been estimated using the conventional Eaton sonic equation with an exponent of 3.0. The observed abnormal pore pressure gradient ranges from 11.85 to 13.10 MPa/km, whereas fracture pressure gradient varies from 17.40 to 19.78 MPa/km. The magnitude of vertical stress (S v ) has a gradient from 21.00 to 23.10 MPa/km. The minimum horizontal principal stress (S h ) magnitude is found to vary from 64 to 77% of the S v in normally pressured to overpressured sediments. A multiple linear regression model with a squared multiple correlation coefficient (R 2 ) of 0.94 is proposed for pore pressure prediction from gamma ray, density and sonic logs to focus on efficient drilling operations and to prevent borehole instability. The statistical model has been calibrated with the RFT data from five wells covering about 3400 sq. km area of the onshore K-G Basin. The model predicted pore pressure values are in close agreement with the actual RFT data for another four wells including a well in the offshore K-G Basin. Hence, the proposed regression model may be useful for predicting pore pressure from other well logs in the K-G Basin.

Overpressure Zones in Relation to In Situ Stress for the Krishna-Godavari Basin, Eastern Continental Margin of India: Implications for Hydrocarbon Prospectivity

“…The abnormal high pressures or overpressures, occurring mostly in the sediments from Cretaceous to Miocene age, have 4 km of thickness deposited in fluviodeltaic conditions implying for a relatively high rate of sedimentation. The double overpressured configuration lies primarily in the wells 5, 6 and 7 located near the Rangapuram field, Amalapuram area (Singha and Chatterjee 2014). The single overpressure configuration lies primarily in the West Godavari sub-basin.…”

“…clay prevents the escape of pore fluids at rates sufficient to keep up with the rate of increase in vertical stress (Singha and Chatterjee 2014). The pore fluid begins to carry a large part of the load and pore fluid pressure will increase.…”

“…The pressure estimate from the Eqs. (2) and (3) had already been compared to the pressure measured by RFT and MDT tools for onshore and offshore wells respectively (Singha and Chatterjee 2014;Chatterjee et al 2011). Previous author e.g.…”

“…The PPG defines the lower limit of MW while FPG defines the upper limit of MW (cf. Tan and Willoughby 1993;Wang et al 2008;Singha and Chatterjee 2014). The abnormal high pressures or overpressures, occurring mostly in the sediments from Cretaceous to Miocene age, have 4 km of thickness deposited in fluviodeltaic conditions implying for a relatively high rate of sedimentation.…”

“…There are several factors for generation of overpressure in sedimentary basins such as: tectonic compression, hydrocarbon generation, disequilibrium compaction, aquathermal expansion, mineral transformation, mineral dehydration, hydrocarbon buoyancy and vertical fluid movement (Osborne and Swarbrick 1997;Bowers 2002;Zhang 2011;Singha and Chatterjee 2014). Precise velocity determination is therefore crucial in geopressure prediction (Dutta 2002;Chopra and Huffman 2006).…”