Error Analysis in Reservoir Evaluation from Surface Pressures

Tue, 1 Aug 2006

Pressure Transient testing is essential for efficient reservoir management.?? However, in many instances it is not practical or possible to acquire down-hole pressure data.? Reasons include excessive cost, high pressures, high temperatures, highly deviated well bores, coated tubing, acid gases or well-bore restrictions from lost tools or "cork screwed" tubing.? In these instances, the SPIDR well testing system is often used to acquire surface pressure data which is then converted to down-hole conditions.? Questions frequently arise over the validity of the conversion of surface data to reservoir conditions.

During the course of its 20+ year history, Data Retrieval Corporation has simultaneously conducted hundreds of pressure transient with down-holes gauges.?? Analyses were then performed on the sensitivity of the WHP-to-BHP model to errors in the input data.? What has been learned, is that since pressure transient analysis is the study of reservoir pressure response to changes in production rate, the absolute values of calculated BHP were relatively unimportant.?? The ability to accurately determine the changes in BHP due to flow rate change was extremely important. ?

As an example, a 17,000 foot deep Gulf of Mexico gas condensate well flowing at 15 million cubic feet a day with a flowing tubing pressure of 4,500 psia was studied.? Input parameters for the error analysis included gas gravity, gas flow-rate, liquid yield, temperature and tubing roughness.? When compared to the actual down-hole gauge measurements, deviations in the input parameters had limited impact on values for skin and permeability but could have significant impact on calculated reservoir pressure.? A 25% error in gas gravity resulted in a 170 psi error in reservoir pressure but had minor effect on skin and permeability while a 50% error in tubing roughness generated a 10% error in skin and permeability.? A 10% error in flow rate resulted in a 10% error in skin and a 25% error in permeability.

This study demonstrated that skin and permeability can reliably be determined from surface pressure measurements, even with uncertain input data, but the accuracy of reservoir pressures determined from surface measurements is dependent on quality input data.