A new technique for forecasting pore pressure in two oil wells in North Rumaila field based on specific energy concept

Abstract

Forecasting the pore or formation pressure is crucially significant in every well drilling operation to determine whether the pore pressure gradient is abnormal or subnormal. Abnormal pore pressure gradient means that the pore pressure gradient exceeds the gradient of the normal pressure, while when the opposite happens it means that the pore pressure is subnormal. Both cases required high care with special control of the mud weight to overcome the vital situation. Accurate pore pressure determination is rigorous in drilling engineering to plan for drilling hydrocarbon wells with appropriate mud program with less effort and cost. Precise forecasting of formation pore pressure prevents occurring various drilling hazards such as lost circulation, stuck pipe and well kick as a result of abnormal pore pressure. In the present work, a new technique was proposed to predict the pore or formation pressure from the specific energy. A new formula of specific energy was used involving the rock properties and the drilling parameters. The new specific energy formula is functioned later in Eaton’s equation to obtain a new suggested formula of pore pressure. There is a lack of researches in the literature that determine the real-time pore pressure without depending on well logs. Abnormal and subnormal pressure zones can be determined accordingly based on the fact that abnormal pressure intervals possess low effective stress and require less energy to drill than the intervals that have normal pressure at the same depth. The new proposed formula of pore pressure was tested in two oil wells in North Rumaila field (N14 and N15) in Basrah province south of Iraq. The obtained pore pressure from the new technique based on the suggested specific energy formula that involves the physical properties of the rock being excavated and the drill bit is compared with the actual (measured) pore pressure derived from other wells in North Rumaila field using measurements while drilling logs, drill stem test and real formation test. There was a good rapprochement between the predicted and the measured pore pressure. The present approach depends mainly on the value of the slope (m) which is determined and varied from one place to another. The new method could provide an alternative method for estimating the pore pressure especially for wells being planned to be drilled where the actual pore pressure is unknown and there is shortage in well logs and formation tests, where most of the previous articles in the literature depend mainly on well logs of adjacent wells in predicting the pore pressure of a specific well. The findings of this study can help for better understanding the prediction of formation or pore pressure that helps to choose the appropriate mud weight to control the well without collapsing and preventing well kick that might occur.