Water Coning In High Permeability Sandstone Reservoirs In Gulf

Abstract

Abstract A field coning test in a high permeability sandstone reservoir in the Arabian Gulf has been conducted. The main objective of the test was to investigate the cause and mechanism of water production. Additional objectives were to obtain a representative value for the vertical to horizontal permeability ratio, determine a representative set of relative permeability curves for reservoir simulation and investigate water recession process after shut-in. A year long test showed that insignificantly looking shale layers imbedded in a clean main sand can pull water fingers laterally, totally preventing bottom water coning. The field test showed that water level dropped about 20 ft in four months of shut-in. A reservoir simulation model was successfully used to design the test and determine logging times. The simulator matched the water cut performance and cone recession time with small changes in the original data. Introduction Water breakthrough is a major concern for naturally flowing oil wells in the Arabian Gulf. Oil production from these wells continue to decline with increasing water production. At around 50 % or less water cut levels, artificial methods become mandatory to sustain the production. Arrival of water to the perforations is often debated among reservoir engineers. Bottom water coning and fingering are the most common occurrences. In sandstone water drive reservoirs, it is important to know at what rates the wells will cone and when will be the breakthrough times. Such knowledge is critically important for planning field wet crude handling facilities and gas lift timing. The same knowledge is important for maximizing production at minimum cost by avoiding severe rate restrictions, extensive workovers and drilling replacement wells. The literature contains a significant number of publications for simplified equations predicting the critical rate, breakthrough times, and water cut behavior after breakthrough (Ref 1). These equations have been derived for idealized reservoirs with uniform vertical and/or horizontal rock properties. While these correlations may work for idealized reservoirs with homogeneous rock properties, their validity for thick heterogeneous reservoirs of the Arabian Gulf is questionable. In the absence of meaningful correlations for real reservoirs, field pilot tests provide reliable information on coning and cusping behavior. The main objective of this work was to conduct a real field test to study water production mechanism, breakthrough and cone recession times. Additional objectives were to determine representative relative permeability curves, study hysterisis effects and obtain a representative value for the vertical to horizontal permeability ratio. Because the giant oil reservoirs in the gulf show mixed wettabilities, such a field test would also shed light on the effect of hysterisis and its significance. Also, data obtained from the test can be used in building full field reservoir simulation models. The following section describes the reservoir chosen for the test. Reservoir Description and Drive Mechanism The subject reservoir is a giant sandstone reservoir interbedded with shales. The main producing zone has an average thickness in excess of 150 ft and an average permeability of 4000 to 5000 md. Relatively tight stringer sands are situated above the main zone and has small thickness. The reservoir has produced in the past 50 years under strong water drive which maintained the reservoir pressure above its bubble point pressure. Reservoir crude shows viscosity variation of 2 to 7 cp and very little solution gas (100 to 300 SCF/STB). The field water cut shows continuing rise due to strong water encroachment. Wells die between 30 to 50 percent water cut depending on the location. P. 381^