Deep Offshore Fracture Pressure Prediction in the Niger Delta - A New Approach

Abstract

Abstract Drilling in deepwater is becoming more and more expensive. There is need to know before hand, for well planning purposes, the values of the fracture pressure. The Fracture pressure could be determined by predictive or verificative methods. Many correlations exists that can be used to predict the fracture pressure. However, they are mostly limited to onshore and shallow water fields. There is therefore the need to develop correlations that can reliably predict the fracture pressure for deep water fields. This paper explores the concept of developing and possibly modifying some existing correlations to suit deep offshore needs as well as develop an Excel Spreadsheet and a V-Basic program that can be used to predict fracture pressure for deep offshore fields both in the Niger Delta region and the Gulf of Mexico. Furthermore, the predicted fracture gradient (LOT) values yielded by the various correlations were compared with actual LOT from a particular deep offshore field in the Niger Delta and the results analyzed. The results of the analysis showed that some of the correlations studied, especially Matthew & Kelly, can be used to reliably predict the facture gradient for deep offshore fields. Introduction Fracture pressure is the minimum pressure required to overcome the formation pore pressure and the strength of the rock matrix as to induce fractures in the formation. Thus, a knowledge of the pressure at which formation fracture will occur at all depths in the well is essential for well planning and drilling operations, as well as for well stimulation and injection operations in secondary recovery.9 Much has been written on the subject of formation pore and fracture pressure gradient estimation. The methods for determining 'geopressures' could be classified as (1) Predictive methods and (2) Verification methods.3,7,18 The term geopressure includes pore pressures, fracture pressures and overburden pressures. For drilling operations, these pressures are most often expressed in the form of a gradient that can be compared with the drilling mud density. Initial well planning is based on formation fracture data obtained by a predictive method. After casing is cemented in place, the anticipated fracture resistance of the formation just below the casing shoe must be verified by a pressure test before drilling can be continued to the next planned casing depth. Such pressure tests includes leak off tests (LOT) and Formation integrity Test (FIT).