An Effective Openhole Horizontal Completion System for Multistage Fracturing and Stimulation

Abstract

Abstract This paper describes an efficient multistage horizontal openhole (OH) completion technique as an alternative to conventional openhole or cemented and perforated lateral completions. The application focuses on OH completions in the Edwards Lime of South Texas. Horizontal wells have been drilled extensively in this deep high temperature gas field. This paper will detail the new horizontal completion system run as part of the production liner, which does not require cementing and provides mechanical diversion at specified intervals, so fracturing and stimulations can be pumped effectively to their targeted zone. Details of the engineering design and testing will be specified, with extensive elaboration on the applications and case histories were these systems have been successfully deployed. The case histories will detail the operational efficiencies of the system in conjunction with the enhanced production realized. Introduction The Edwards Lime in which the primary target lies is composed of limestone, parts of which are dolomitic and cherty. The top of the Edwards Lime is marked by a dramatic change in drilling from the extremely hard Georgetown to the extremely soft drilling of the upper 2–3 feet (ft) of "rotten" dolomitic limestone, designated as the top of the Edwards Lime. Beneath this "dobe" as some of the local drillers have referred to it is 8 - 10 ft of soft dolomitic porous limestone. This is commonly referred to as the Edwards "A" zone. Beneath this is a 6 - 8 inch (in) concretionary flint layer that generally marks the division between the Edwards "A" and the Edwards "B". The top of the Edwards "B" is characterized by a 15–20 ft thick section of very soft, porous, dolomitic limestone. Both the Edwards "A" and the Edwards "B" have been productive in the past in Salt Flat field. Edward limestone produces gas in a field called Word Field at an approximate depth of 13,000 ft. The Edward limestone surface dips southeast with the shelf edge evident by a sharp break. The production generally comes from the upper 300 ft. of Edward and up dip from the shelf edge. The porosity in the limestone is believed to be secondary in nature and observed near the shelf edge. Secondary porosity is however, not uniformly distributed away from the shelf edge. Although economic porosity varies from well to well, an average porosity of 6% over the top 300 ft is a good cut off number. Wells with average porosity above 8% are considered to be good successful wells. Some wells less than average 6% do produce from higher but very thin porosity zones within the upper part, but normally they are very poor producers from the outset. Productivity in the Edwards Lime has historically been based on vertical stimulated wells due to the low horizontal permeability and the inability to control fracture placement in the OH horizontal wellbore. Additionally, the low stress barriers in the bottom of the Edwards, below the Edwards "B" section allow fracture growth to extend down into the aquifer resulting in water production. This has been documented for over 30 years with various fracture and stimulations designed around this fact. The move toward horizontal wells presented new challenges in wellbore construction and stimulation. In cased, cemented 0 and perforated applications, the balancing of economic and mechanical risks often proved more costly than vertical wells. For OH completions the results were often the same as cased hole, with the exception of costs and risks being significantly lower. Thus, the opportunity for the Edwards Lime was to develop an OH completion system that would allow fractures to be placed and controlled at strategic points along the horizontal wellbore. Typical Well Configuration and Completion Methods Most projects have traditionally followed a vertical well configuration that was cased, cemented and perforated, then stimulated with various types of acid treatments or proppant fractures to promote production. Stimulation results have historically provided very short term benefits. OH completions utilized single stage acid treatment with diverters. The shallow upper section of the Edwards formation, coupled with its depth and bottom hole temperature (BHT) made the vertical wells typically attractive, provided the fracture or stimulation could be controlled to prevent water production. Horizontal drilling showed promises of improving economics of the play, but the ability to effectively stimulate the horizontal wellbore proved challenging and initially drove the economics below that of vertical offset wells. Drilling optimization over the last several years combined with horizontal completion and stimulation advancements have made the Edwards Lime a very viable and economic resource.