GEOMECHANICAL MODEL FOR WELL BORE INSTABILITY PREDICTION ‘BADE’ FIELD ONSHORE, NIGER DELTA.

Abstract

The study has developed a geomechanical model for well stability control and sand production prediction of the reservoir in ‘BADE’ Field. It determined the state of in-situ stress and measured the hazards in terms of stress, pressure, weak and fracture zones as well as geomechanical properties; obtain an adequate geomechanical model to comprehensively explain wellbore instability and determine the drilling fluid properties to minimize risks associated with wellbore instability. This study is motivated by reported instability of wellbores in the study area by evaluating geomechanical properties to which a rock is subjected at depth; characterize the potential drilling geomechanical model using well data which could not provide information away from well locations. This study probes into the possibility of using horizons picked from seismic sections and velocity to extend Mechanical Earth Model to 3D space and provide information on the inter-well spatial variation on porosity, pore pressure, fracture, faulting and wellbore instability within the study area. The study area, an onshore field, is located within latitudes 5°30N to 5°40N and longitudes 6°00E to 6°20E in the Niger Delta. The data base comprised Pre-stack seismic and composite well logs for 6 wells and base map of the study area. Petrel 2009 and Rok Doc software were used for the study. The research work flow included lithology identification, matrix property determination, mechanical parameters and rock In-situ Stress estimation, modelling of the overburden rocks in the zones of interest. The in-situ principal stress, effective stress, pore pressure and rock mechanical parameters were determined using appropriate mathematical equations along the wellbores. Rock strength and integrity analysis were determined for instability prediction analysis in the study area. 3D structural geo-models of these parameters within zones of interest were generated in order to characterize potential drilling hazards. The results revealed that in-situ principal stress magnitudes viiincreased with respect to the lithology depth and density variation trend in order of > > ℎ in all the wells, indicating that a normal stress regime dominated the field. The in-situ stress magnitude is higher in shale formations than reservoir sands. Elastic parameters revealed that shale formation had lower resistance to stress than the reservoir sands. This implied that reservoir sands exhibited high strength magnitude both at the shallow and high depths in the study area. These showed that the dominant shale is weak and easily deformed under high stress as a result of its low strength. The effects of high pore pressure against low cohesion and internal friction angle were found to be capable of exposing the wellbore walls to high risk of instability. It was noted that, it will take high rock strength to prevent rock failure when stress concentration is high within shale formation. The study has been able to adduce reasons for the widespread wellbore instability in the study area to high stress, low strength and low elastic parameters magnitudes on the dominant weak shale units there.