Edwards Limestone Completions Improved By New Stimulation Technique
Authors: Misak Marvin Don , Atteberry Roger D. , Venditto James Joseph , Fredrickson Sherman Edward 
Published: December 2007

Abstract
A major problem in Edwards Limestone completions has been the high production of formation water following fracturing treatments. This is generally considered to be due to the lack of a barrier to prevent downward growth of the fracture from the gas producing zone into the water producing interval. Thus, with conventional treatments the undesirable water producing zone is fractured and propped during the fracturing treatment of the hydrocarbon producing zone.
A density controlled fracturing technique has been developed that is capable of placing the proppant in the upper portion of a producing zone. The technique utilizes a producing zone. The technique utilizes a high density pad followed by a lower density treating fluid containing the proppant. This sequence allows the proppant to be preferentially placed in the upper portion of preferentially placed in the upper portion of a vertical fracture.
Laboratory tests have been conducted with a fracture model that demonstrated layering of fluids of different densities. Field results of several density controlled fracturing treatments conducted along the Edwards Limestone Trend of South Texas are presented. Results demonstrate that this presented. Results demonstrate that this method is capable of increasing hydrocarbon production with little or no increase in the production with little or no increase in the production of formation water. production of formation water

Introduction
The Edwards Limestone Trend underlies a substantial area in South Texas. Hydrocarbon accumulation is in a build-up of reef facies of the formation, and is controlled lay structual relief and an up-dip permeability barrier. The Edwards Formation is found between depths of 8,500 and 15,000 feet, and is approximately 1,000 feet in thickness with hydrocarbon accumulations occurring near the top. The reef rock occurs as a rather unique and very narrow band of porous limestone reservoir rock completely surrounded above, behind, and frontally by dense limes and impervious shales.
The fore reefs are comprised of a monotonous sequence of fine grained, dark gray, dense, deep water limestones. Those behind the reef are light gray to buff, fine grained, miliolid limestones deposited in a lagoonal environment. The reef rock itself is a white to light gray, finely crystalline limestone with numerous large fossil fragments. These fossils are predominantly rudists, corals and algal remains. Core samples show vuggy porosity resulting from incompletely filled porosity resulting from incompletely filled body cavities of rudistid shells. Calcite cement and fine lime mud are common throughout the rock and have greatly reduced the porosity. As the reef surface apparently porosity. As the reef surface apparently represents an erosional surface, the capping rocks may either be deep water limes or dark marine shales, varying in stratigraphic position from Georgetown to Middle Eagle position from Georgetown to Middle Eagle Ford.
The basic reef mass shows a prolonged uninterrupted growth resulting in the buildup of a very uniform lithologic section, averaging 1,000 feet in thickness. Porosities in the core of the reef average 4 to 6% with corresponding permeabilities of 0.01 to 0.5 millidarcies, with the lower values predominating. predominating. It is often necessary to stimulate the Edwards Limestone reservoir formation to obtain commercial hydrocarbon production. However, due to the water producing interval in the lower part of the zone, conventional stimulation techniques normally result in high volumes of produced water. This is due to the lack of a barrier that is capable of preventing vertical fracture growth downward preventing vertical fracture growth downward from the hydrocarbon producing zone into the aquifer.