Thermal EOR Pilot Plan for a Shallow Limestone Reservoir (Rubble Zone) in Bahrain Field

Abstract

Abstract Bahrain field, the first hydrocarbon discovery in the Arabian Gulf, is a multi-stacked elongated anticline structure with a stack up of 16 Oil & 4 Gas Reservoirs. "Rubble Limestone" is the descriptive name given to a massive Limestone unit of the Mishrif formation, in middle Cretaceous in age. The zone consists of two layers, namely Upper Rubble and Lower Rubble. Both the layers are eroded in the crestal part of the structure. Large volumes of oil are trapped against the Blue shale at the up structure pinch outs of upper & lower Rubble. Fault cuts are observed in more wells indicating intense faulting and fractures.. The zone has both heavy & light oil. The amount of Heavy oil is estimated to be 90% of the initial oil in place. Currently only light oil is being produced. This paper highlights the studies to evaluate Heavy oil production potential. Two separate studies were carried out, the first was to describe and characterize Rubble in order to map the light & heavy oil and the water bearing layers. Geo statistical methods were used to populate the 3D model of the reservoir with porosity & permeability values. A detailed analysis of the logs indicated that light oil is associated with large separation between deep & shallow resistivity. It was observed that whenever there was heavy oil, the separation between the resistivity curves was either very less or even negative. Three rock types were identified and the habitat of light, heavy oil and water was accordingly assigned. After obtaining an estimate of the areas of heavy and light oil, a fresh study was initiated to review and evaluate the Heavy oil prospect and to prepare a suitable thermal EOR pilot plan for the heavy oil recovery using Fraca® and PumaFlow® softwares. The study included (a) a review of the Reservoir description done earlier, (b) numerical modeling and simulation of a pilot area, (c) find out the suitable EOR process and design the pilot. The study indicated that the reservoir has too low matrix permeability and very small fracture volumes to allow steam chamber formation. SAGD mechanism was therefore found not efficient for this reservoir. Huff and Puff has been simulated using two wells in the pilot area and found attractive. Cold production from six different Horizontal wells were simulated and tested. They indicated that cold production followed by Huff and Puff will be efficient to recover the Heavy oil. The paper will describe the results of the study which led to drilling of the Horizontal pilot well for Cold production to be followed by Huff and Puff Thermal process.