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Formation Pressure Correction to Enhance the Prediction of the OWC in a Normal Pressure Reservoir


The oil-water contact (OWC) is defined as the depth of bottom oil layer in an oil reservoir. It's especially important that how we determine the position of the oil-water contact when a new reservoir is discovered, but only the pure oil zone is drilled through by the exploration wells. Generally basing on the formation pressure acquired from the wireline formation testing and the theoretical basis of equal pressure at the fluid interfaces, there are two methods to predict the OWC in a normal pressure reservoir: 1) Linear diagram of oil and isobathic hydrostatic pressure VS depth; 2) Semi-closed U-tube manometer physical model. However there are several limitations during the practical application of the two methods. Firstly the first method is set up depended on the assumption of homogeneous fluid density and the predicted depth is actually the free water level which has a gap to the real OWC. Secondly due to the density variation of the crude oil components and compounds they could occur gravitational differentiation which leads to relatively lower crude oil density and viscosity as well as higher formation pressure on the top of the oil column, and that will influence the linear relationship of the pressure VS depth curve resulting in erroneous calculated OWC. Moreover affected by the secondary gas cap sourced from the dissolved gas generated during the hydrocarbon accumulation and migration and crude oil biodegradation, the measured formation pressure is higher than the original one in reservoir. Because of the above limitations, there are different degree errors between the forecasted results and the exact OWC according to reliable information from almost 50 developed reservoirs in BZ oilfield, offshore Bohai Bay Basin in China. The maximum error is even up to 39m. In this study, so as to obtain the correct OWC, we find ways to exclude the effects of reservoir fluid heterogeneity on the linear diagram of oil and isobathic hydrostatic pressure VS depth and recover the original oil pressure by eliminating the gas cap pressure from the measured pore pressure, by which the forecast errors will be reduced. According to the example applications, after utilizing the pressure correction into the two methods, an average forecast error has been greatly reduced to 1.3m in BZ oilfield. Perfectly forecasting the OWC has important significance to estimate the hydrocarbons in place and formulate the more reasonable reservoir development programs.