The American Oil and Gas Reporter - January 2012 - (Page 77)

Oil & Gas Shales Study Optimizes Eagle Ford Completion By Kennedy Chukwuemeka Nwabuoku HOUSTON–A major challenge to efficiently completing a horizontal well in shale plays is determining the optimal stage spacing and number of perforation clusters per stage to effectively drain the reservoir. Because the Eagle Ford Shale play in South Texas is still in its development infancy compared with other North American shale plays, it is lacking scientific studies that address these key lateral coverage questions. In developing the Eagle Ford, industry practices have been based largely on available capital or best estimations. Even though the Eagle Ford has different formation characteristics and mineralogy content, the tendency on initial wells was to apply the same drilling and completion designs used in more established shale plays. El Paso Corporation undertook a project to study stage spacing and numbers of perforation clusters per stage in four of its first Eagle Ford horizontal wells in an effort to optimize completion design and improve reservoir performance. The wells were completed with typical 4,000-foot lateral lengths with four to six perforation clusters and 300,000-350,000 pounds of proppant per stage, with a total of 14-16 stages. Radioactive tracers (antimony, scandium and iridium) were used to tag the proppant, chemical tracers marked the stage frac fluid, and flow-through composite bridge plugs isolated the treated zones. After stimulating all the zones, the plugs were drilled out and a spectral gamma ray tool was run in the hole with coiled tubing to log the lateral. Tracer log results from the first well in the study showed that most of the perforated zones were treated, with the exception of the toe stage, which showed little or no stimulation. A close observation of the tracer log output indicated that the treated lateral area (the area with tracer signature) was less than half of the entire lateral length. A linear count of the exact lateral length covered by the tracer mark compared with the entire lateral length revealed less than 36 percent coverage. After drilling the first well, changes were made to the completion methodology to improve the stimulated lateral coverage and reservoir performance (the well productivity index). These changes resulted in obtaining 100 percent improvement in stimulated lateral coverage, with a corresponding positive impact on productivity index. Understanding The Reservoir The hydrocarbons produced from the Eagle Ford formation range from predominant dry gas in the south (Duval County), to condensate-rich liquid (La Salle County), to predominant oil (Zavala and Frio counties). Figure 1 shows the general areas where the reservoir produces oil (green), high liquids (dark green) and predominately dry gas (brown). Moreover, some parts of the play are overpressured, and the basic structural characteristics of the shale vary significantly across the play. For example, gross height ranges from 20 to 500 feet in thickness, and depth varies from 2,500 to 14,000 feet. The variability makes understanding the local reservoir critical to successfully developing and stimulating the Eagle Ford. The first exploratory Eagle Ford wells were drilled in the dry gas window in FIGURE 1 Eagle Ford Dry Gas, Condensate and Oil Windows LaSalle County in late 2008. The early completion designs emulated Barnettstyle water frac stimulation treatments and had varying degrees of success. Determining what spacing to apply and how many perforation clusters to use per stage are issues that many operators have struggled with. With Eagle Ford wells costing $8 million-$10 million, it is important to achieve efficient lateral coverage without spacing fractures too close together. Hydraulic fracturing is the largest single completion cost component, and spacing determines how many fracs will be placed in a given lateral length (the denser the spacing, the higher the cost). The effectiveness of the fracture network created depends on multiple factors. Given the same fracture design (job size, rate, fluid type, etc.), the major variables are spacing between composite bridge plugs, the number of perforation clusters per stage, and the space between the clusters. Considering the ultralow permeabilities of shale formations, the amount of hydrocarbons drained from the reservoir are proportional to the stimulated rock volume (SRV), making placing multiple, JANUARY 2012 77

Table of Contents for the Digital Edition of The American Oil and Gas Reporter - January 2012

The American Oil and Gas Reporter - January 2012
Oil & Gas Counts
State Legislative
Industry Digest
Tech Connections
Washington Watch
Federal Legislation
Gulf of Mexico
Resource Plays Providing Wealth of Opportunities
Tight Plays Poised to Transform U.S. Crude Supply
El Paso Project Optimizes Eagle Ford Completion Design
Real-Time Forward Modeling Improves Bakken Horizontals
Cover Story
Financial Firms Expand Oil and Gas Divisions
Multicomponent 3-D Poised for Growth in Shale Plays
Cloud Computing Driving Business Step Changes
Advanced Attributes Improve 3-D Interpretation
3-D Data Improve Knowledge of Shale Heterogeneity
HBUR RSS Solves Granite Wash Drilling Challenges
Technologies Improve Production Consistency in Resource Plays
SaaS Helps Operator Streamline Data Management
Automation Enhances Operations in Challenging Applications
Dispersant Chemistry Combats Plugging in Low-Gravity Oil Wells
New Technologies Optimize Production
Drilling Regs
The Presidential Papers
Energy Education
Shale Gas
New Lits & Products
Computer Currents
Industry Focus
Classified Advertising
Advertisers Index

The American Oil and Gas Reporter - January 2012