Case History: Pushing The Limit On Long Horizontal Gravel Packs
Authors: Attong David J. , DeLandro Wendell V.C. 
Published: December 2007

Case History: Pushing The Limit On Long Horizontal Gravel Packs.

Abstract
This paper represents dramatic progress in horizontal completion technology by Amoco Trinidad Oil Company by successfully gravel packing two wells of approximately 2,500 ft lateral 8-1/2 in. hole sections. The target reservoir was normally pressured at a depth of 7300 ft. TVD. In one well a perforated liner was run prior to running prepacked screens and gravel packing, in the other only prepacked screens were run. In both cases water was used as the carrying fluid contrary to the conventional area practice of pumping gel slurry packs. To date both wells are approaching one year of sand-free production. Based on this achievement our ability to gravel pack long horizontal 8-1/2 in. sections should not be the limiting consideration in an extended reach drilling program. Worldwide, these cases can lay claim as two of the longest gravel packed horizontal sections to date.

Introduction
Amoco Trinidad Oil Company's area of operations lie off the southeast coast of Trinidad (Fig. 1) where commercial hydrocarbons occur in sandstones of Pliocene age. Geology of the area is characterized by unconsolidated reservoirs requiring sand control techniques. Since initial production in 1971, sand control was and continues to be a challenge which has always been successfully met, in vertical and deviated wells. Gravel placement is successfully accomplished by using gel slurry as the transport fluid in most completions. Experience and initial early testing has shown that 40-60 US Mesh rounded gravel and 8 gauge prepacked screens can successfully stop sand production in this area. In the past field tests using water packs were tried with very limited success. Interval lengths of up to 350 ft. have been successfully gravel packed in either cased or open hole in reservoirs with varying bottom hole pressures. When Amoco Trinidad embraced horizontal technology we concentrated on our ability to drill the well and less emphasis was placed on completion. The geological risks were foremost in our planning and preparations, in fact, the length of the lateral section was determined by geological information obtained while drilling. The challenge to gravel pack and complete wells was set by how well we could drill the lateral sections. Initial tests were already done to determine the feasibility of gravel packing one horizontal annulus. Further tests were conducted to determine the best method of packing two annuli using 30 ft. horizontal model. Combinations of gel and water were used as transport fluids along with gravel and plastic beads. The best results were obtained using a water pack and rounded gravel. One major completion concern was would the screens tend to become plugged while running in the lateral section? There was no conclusive evidence to indicate the degree of plugging if any. Diagnostic tools to evaluate gravel packs are limited in horizontal wells, in our cases no gravel pack logs were run and our success was measured purely on volume of gravel placed and sand-free production.

Well Design
The typical wellbore trajectory was designed to set 9-5/8 in. casing at 90 degrees inclination just at the top of the target, which was a 4 - 10 ft. TVD window in the sand. Typical build up rates were approximately 6 degrees per 100 ft. After setting casing the 9.5 ppg fresh water Low solids non-dispersed mud was displaced to a 9.6 ppg proprietary polymerized calcium carbonate drill-in fluid to drill the 8-1/2 in. lateral section, this fluid contains graded calcium carbonate which deposits a nondamaging filter cake that is easily and effectively removed by initial production. After drilling the lateral section the wellbore was displaced to 9.5 ppg FSW + 2% KCL completion fluid. At this point prepacked screens were run in the hole prior to water packing. If the lateral section contained a predominant shale break or a high percentage of shale, serious consideration was given to running a perforated liner to increase the chance of gaining access to the entire lateral section. The prepacked screens were then run and gravel packed.
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