Modified Eaton’s Sonic Model for Improved Prediction of Pore Pressure in a Shaly Formation

Abstract This study presents the development and validation of a modified Eaton’s sonic model for more accurate pore pressure prediction in shaly formations, using shale compaction trends derived from the normal compaction transit time equation. The modified model integrates shale transit time data to estimate porosity, bulk density, and overburden pressure—key parameters for predicting pore pressure in complex geological settings. Validation was carried out using Repeated Formation Test (RFT) data from a Gulf of Mexico deep-water well at 20,000 ft depth, where the observed shale transit time was 108 μs/ft, the normal trend line was 68 μs/ft, and the compaction equation transit time was 70.5 μs/ft. At this depth, the overburden pressure was 17,680 psi, and the RFT-measured pore pressure was 15,392 psi. The modified Eaton model predicted a pore pressure of 15,293 psi with a percentage error of 0.6432%, compared to 15,538 psi from the original Eaton model (0.9485% error) and 15,526 psi from the Bower model (0.8706% error), showing improved accuracy. At 19,000 ft, the modified model estimated 14,887 psi compared to 16,965 psi from the original model, further demonstrating improvement. Additional data from a South Louisiana Miocene well showed how shale transit times supported accurate estimations of porosity and bulk density, with porosity decreasing from 0.2483 at 4,500 ft to 0.08966 at 10,500 ft, and bulk density increasing from 2.260 g/cc to 2.509 g/cc over the same interval, confirming expected compaction trends. The study supports safer drilling by improving casing depth and mud weight decisions, keeping mud weight within safe pressure limits. Its higher accuracy makes the model a valuable tool for pore pressure prediction in shaly formations.