Determining mud weight window to overcome wellbore instabilitywith case study Fula North Field in Sudan

Abstract

Wellbore stability-related problems are one of the main sources of time and money losses in any drilling operation. This is the reason why, in the last years, many improvements have been made in this specific field of research. The improvements are based on a better knowledge of the mechanisms that originate rock failure. One of the key parameters to be controlled and monitored while drilling is that of the mud weight. In order to avoid well instability, a graphical representation of the safe mud weight window is presented. This graphical solution is intended to ease the estimation of the values that guarantee well stability, either in tension or compression, at any depth, based on the geomechanical properties of the formations crossed while drilling. During drilling operations for the Baleela oil field in the Abu Gabera formation in Sudan, loss circulation has been identified as a geomechanical problem for several wells. In this project a one-dimension geological earth model of the Abu Gabera formation is compiled based on its state of stress and rock strength parameters. The mechanical earth model depends on the principle in-situ stresses which obtained from wireline logging. Rock strength properties obtained from empirical equations. Therefore, we can minimize non-productive time NPT and the cost of drilling significantly by precluding some drilling problems. Based on the MEM results, the mud pressure window is calculated and a mud weight is recommended for the Abu Gabera formation. The field case provides proof that the Matlab software is a very good tool for predicting a safe mud weight window.