No abstract

Hydraulic fracturing of wells in naturally fractured reservoirs can differ dramatically from fracturing wells in conventional isotropic reservoirs. Fluid leakoff is the primary difference. In conventional reservoirs, fluid leakoff is controlled by reservoir matrix and fracture fluid parameters. The fluid leakoff rate in naturally fractured reservoirs is typically excessive and completely dominated by the natural fractures. Historically, attempts to fracture-stimulate wells in naturally fractured reservoirs have been unsuccessful due to high leakoff rates and gel damage. The typical approach is to attempt to control the leakoff with larger pad volumes and solid fluid loss additives. This approach is not universally effective and can do more harm than good. This paper presents several field examples of a fracture stimulation program performed on the naturally fractured Devonian carbonate of West Texas. Qualitative pressure decline analysis and net treating pressure interpretation techniques were utilized to evaluate the existence of natural fractures in the Devonian Formation. Quantitative techniques were utilized to assess the importance of the natural fractures to the fracturing process. This paper demonstrates that bottomhole pressure monitoring of fracture stimulations has benefits over conducting minifrac treatments in naturally fractured reservoirs. Finally, the results of this evaluation were used to redesign fracture treatments to ensure maximum productivity and minimize costs.

Technology Focus It is important to put the role of well stimulation into perspective as key to driv-ing production and competing in the world's energy market. Interesting information can be found in the US DOE Energy Information Agency's report Table 11.1 World Primary Energy Production by Source, 1970–2005, which is available at http://www.eia.doe.gov/aer/txt/ptb1101.html (accessed 7 April 2009). Oil last peaked as the world's primary energy source in 1973 at just more than 47% of all sources. By 2005, this number had dropped to approximately 34%, although the world's total energy production had nearly doubled over these 32 years. While coal remained fairly steady at 25 to 27% of the world's market share, natural-gas and natural-gas-liquid (NGL) usage increased from 20 to 25%. This suggests that natural gas could surpass oil and coal as the world's primary energy source within the next decade or so, unless low natural-gas prices continue. Renewable-energy sources (e.g., nuclear, hydroelectric, and geothermal) gained an increase in total market share from 7 to nearly 14%, with these sources supplying more than 63 quadrillion Btu to the world in 2005 alone. Natural gas and NGLs had not surpassed this level until 1984, just 21 years earlier. Renewable energy has doubled its market share during a doubling of the market size. To improve recovery, well-stimulation tests, data, and interpreters need to become valuable commodities on par with the other well costs. Data and analysis provide a basis for improvement. Whether success may come from higher retained conductivity, comparing pre- vs. post-treatment buildup tests, or acquiring bottomhole treating pressures to unmask friction effects, many avenues are available to enable us to compete at the next level. We need to embrace our best practices, strive for new developments, and analyze much more data to perform at the next level. Without measuring results, we cannot prove the improvements. Although oil may be nearing its end as the world's primary energy source, it is doing great as a source of products. To transition to natural gas as the new energy leader, well stimulations will be a key to keeping the petroleum industry as the world's best provider of clean and cost-effective energy. Please enjoy reading the following selections representing the future of well stimulations and interesting examples of how we can compete. Well Stimulation additional reading available at the SPE eLibrary: www.spe.org SPE 119623 • "Acid Fracturing: The Effect of Formation Strength on Fracture Conductivity" by A.M. Gomaa, SPE, Texas A&M University, et al. SPE 121483 • "First North Sea Application of Pinpoint-Stimulation Technology To Perform a Rig-Based Acid-Fracture Treatment Through CT" by David Barclay, SPE, Halliburton, et al. SPE 116775 • "An Innovative Acid-Stimulation Technique for Reviving Dead Wells in the Ghawar Field of Saudi Arabia—A Holistic Approach" by Surajit Haldar, SPE, Saudi Aramco, et al.

Technology Focus This feature highlights three papers that are examples of working smarter, better, or cheaper to help us be more competitive in an ever-growing world energy market. In 2004, oil and gas production was the leading supplier of energy generation with approximately 55% of the world market. Four key competitors made up more than 44%: coal, nuclear, hydroelectric, and bio-mass/waste. It would seem that new competitors continue to appear. One major E&P company is touting its lead as the largest producer of magma-heated steam to drive power generators—enough to support millions of homes. The water captured after power generation is reinjected and reheated to temperatures as high as 570°F, completing a renewable-energy cycle. Then there is the latest liquid-sodium-cooled nuclear power plants, such as the "Toshiba 4S." Additionally, others are pushing solar, wind, bitumen, and now power plants that can switch between coal and natural gas. As the price of oil climbs, our competitors will catch up all the quicker to meet the world's growing energy needs. Now is the time to raise the bar for defining well-stimulation success, not after our competitors have caught up. Is it possible that we spend too much time looking at future wells and not enough quantifying past results? Let us take the example of stimulating a four-fold increase in production, which generates a high economic return. If we determine that we could have achieved a six-fold increase with a greater return, or perhaps even a 20-fold increase by combining with a multistage-treated horizontal well, does this alter our idea of success? It is time to take stimulation effectiveness to the next level, being measured and understood more reliably through integrated and detailed approaches. Success should be demonstrated through consistent engineered improvements. It is defining a greater understanding from previous stimulations that will allow us to realize more rapidly what is possible, thereby increasing our level of competition in a world market, one well at a time. Well Stimulation additional reading available at the SPE eLibrary: www.spe.org SPE 110707 • "Real-Time Diversion Quantification and Optimization Using DTS" by Gerard Glasbergen, SPE, Halliburton, et al. SPE 111512 • "Innovative Water-Shutoff Solution Enhances Oil Recovery From a West Venezuela Sandstone Reservoir" by Goran Andersson, PetroBoscan, et al.

For this month's issue of featured well-stimulation papers, the ongoing work in acidizing earns my recognition as the most enlightening and yet overdue progress. Although the papers spotlighted in this issue are strong presentations on diverse subjects, I was most surprised by the volume of engaging papers focusing on acids and fracture acidizing. There are great papers in my list with information on horizontal wells, gravel packing, and viscoelastic fluids; however, each of these technologies is young compared to acidizing reservoir rock. In one of the additional-reading selections, "Fracture Acidizing: History, Present State, and Future," we learn that acidizing carbonates dates back to 1895. Therefore, the use of acids to increase production is more than a century old. With many of the modern acidizing theories developed around 1972 by Neirode and others, it is still a 35-year-old technology that needs a fresh look with modern methods. Whether we are studying etching behavior to forecast conductivity or predicting long-term reductions in performance caused by creep, we still have a lot to learn about this venerable practice. Therefore, the question becomes: "How can acidizing oil wells be more than 100 years old, yet we just now are beginning to unravel these fundamental concepts?" The world produces approximately 85 million BOPD, and assuming just USD 50/B, we generate more than USD 1.5 trillion/yr in world oil revenue. Whatever the historical disconnect has been between oil revenues and funding oil science, I am pleased to recommend these compelling technical papers on subjects vital to our industry, including the common practice of pumping acid into a formation to increase production. Well Stimulation additional reading available at the SPE eLibrary: www.spe.org SPE 108075 "Horizontal-Well Completion and Stimulation Techniques—A Review With Emphasis on Low-Permeability Carbonates" by Valdo Ferreira Rodrigues, SPE, Petroleo Brasileiro, et al. SPE 107772 "The Effects of Acid Contact Time and the Resulting Weakening of the Rock Surfaces on Acid-Fracture Conductivity" by M.G. Melendez, Texas A&M University, et al. SPE 107760 "Acid Stimulation of Extended-Reach Wells: Lessons Learnt From N'Kossa Field" by J.M. Mazel, Total, et al. SPE 106371 "Fracture Acidizing: History, Present State, and Future" by Leonard J. Kalfayan, SPE, BJ Services