Pore pressure prediction and fracture gradient determination using compressional and shear velocities from sonic log (P-1110) (MFN 3647)

Abstract

The prediction of pore and fracture pressure along a proposed well path are the key inputs to well design. Accurate predictions of these parameters can have significant impact on the cost of wells, both from design and operational perspectives. There are many complications associated with abnormally high fluid pressures in overpressured formations. Pore pressure can directly influence all parts of operations including drilling, geological studies, completion, and production. Accurate predictions of pore pressure and fracture pressure are vital aspects to the production and completion of safe, time efficient, and cost efficient projects. Knowledge of pressure distribution in the formation can greatly reduce complexities associated with drilling and completing a well. The purpose of this project is to develop a Pore Pressure and Fracture gradient prediction strategy for the Meyal Field. Petrophysical, borehole acoustic, and drilling parameters for the well Meyal-16 previously drilled in the Meyal area is examined and reviewed. The pore pressure and fracture gradient prediction strategy will be useful when designing future drilling and completion operations in the aforementioned area. Pore pressure and fracture gradient prediction strategies are reviewed and applied to the available data. The pore pressure prediction strategy reviewed is developed by ―Ben Eaton‖. Eaton developed a simple relationship that predicts the formation pore pressure knowing the normally pressured compaction trend line and observed sonic log data and a relationship for formation overburden stress. The fracture pressure prediction strategy reviewed is also developed by ―Ben Eaton‖. The data required for this prediction strategy is formation overburden stress, pore pressure and formation Poisson's ratio. A relationship for the overburden stress and Poisson's ratio can be developed or one of Eaton's published relationships can be used. Ultimately, the Eaton fracture gradient prediction strategy results in a simple and accurate relationship provided an accurate estimate of pore pressure is available. Using the petrophysical and borehole acoustic data, the resulting formation pore pressure predictions of the Meyal-16 well by the Eaton pore pressure prediction method was chosen as the best model to implement in future operations. The fracture gradient prediction method is implemented using the formation pore pressures estimated by the Eaton pore pressure method. Pore pressure and fracture pressure prediction is complicated by abnormal pressure in the formation due to undercompaction and seals. Low permeability and traps prevents fluid from escaping as rapidly as pore space compacts thus creating overpressure. Results of the study on Meyal-16 clearly indicate that the mud weight of 15.3 ppg used in drilling the reservoir is on the higher side which resulted in the breakdown of the formation. Best suitable mud weight is found to be within the range of 9 ppg to 11 ppg using the Eaton method. The fracture gradient computed from Eaton method ranges from 11 ppg to 12 ppg an average. This is the fracture initiation pressure of the formation at which formation will be fractured.