Surface Calculated Pressure Buildup Testing in the Kuparuk River Unit, North Slope Alaska
Authors: Ramirez A. R. , Renfro K. D. 
Published: December 2007

The use of surface calculated pressure buildup (SCPBU) tests has proven to be an accurate and cost effective technique for acquiring pressure transient data in the Kuparuk River Unit on the North Slope of Alaska. The technique involves measurement of the surface tubing pressure and use of a computerized acoustic device to measure the depth of the gas/liquid interface in the tubing of a shut-in production well. The bottom-hole pressure is then calculated by applying the gradients of the respective fluid columns. Conventional pressure transient analysis is then undertaken which provides useful information regarding provides useful information regarding reservoir and completion characteristics.

The equipment used was developed for performing SCPBU's in rod-pumped wells by performing SCPBU's in rod-pumped wells by monitoring the casing pressure and fluid level. However, with minor adaptations, it is successfully being employed at Kuparuk to measure the downhole pressure buildup via the tubing string of producing wells. To avoid the potential of natural gas accumulation in the wellhouses which protect wellheads in the arctic, the acoustic signals are generated by explosion signals from a nitrogen bottle secured in the wellhouse rather than through implosion to the atmosphere. As these acoustic signals are reflected by downhole jewelry such as gas lift mandrels (GLM), additional attention is required to ensure that the computer does not mistake a GLM for the fluid level as the fluid level rises or recedes in the tubing. This is currently being accomplished by frequent on-site monitoring of the fluid level's location in relation to downhole equipment, and adjustment of the "mute time" after which the computer will recognize an acoustic reflection as the true fluid level. After determining the measured depth to the FL, the computer then uses Input well deviation data to determine the true vertical height of the gas and liquid columns in the wellbore. The bottom-hole pressure is then calculated by applying the gradients of the respective fluid columns, with conventional pressure transient analysis techniques applied to the resulting pressure transient data.

It should be noted that using the subject technique presents an additional unknown in many pressure transient calculations in that the flowing bottom-hole pressure (FBHP) must be estimated instead of measured.In most situations at Kuparuk, proven hydraulic correlations allow the FBPH to be assumed within a range suitable for analysis of most pressure transient parameters, such as skin, pressure transient parameters, such as skin, permeability, and flow efficiency. permeability, and flow efficiency. In the past year, over 50 SCPBU's were completed on wells in the Kuparuk River Unit. Of these tests, over ten were measured simultaneously with downhole pressure measurement devices. These concurrent tests provide a means to evaluate the accuracy of provide a means to evaluate the accuracy of the surface calculated measurement technique through direct comparison with the current industry standard of downhole measured pressure transient testing. Results of these pressure transient testing. Results of these concurrent tests indicate close agreement after the dynamics of phase separation in the tubing have taken place. In Kuparuk wells, the first 4-8 hours of shut-in time is a period marked by the dynamic migration of period marked by the dynamic migration of natural gas from the live oil present in the wellbore and reservoir, resulting in a constantly changing fluid gradient in the wellbore. Due to the numerous complicating factors involved, a correlation to accurately model the changing gaseous-liquid gradient has not yet been developed for Kuparuk.

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