Natural-gas production from tight and shale-gas reservoirs will be increasingly important in China as the country shifts from coal-based energy to cleaner energy sources. Recent Chinese sources have estimated that the gas-in-place resources from tight and shale-gas reservoirs in China are at least 12×1012 m3 and 31×1012 m3, respectively. In 2008, annual production from tight gas reservoirs reached 20×109 m3, approximately 23% of the natural-gas production in China. Commercial production from shale-gas reservoirs has not begun but is expected to grow rapidly.
Introduction
Since 2010, the consumption of natural gas in China has far exceeded domestic production. To bridge this gap, domestic production of natural gas, especially from unconventional resources [e.g., coalbed methane (CBM) and tight and shale-gas reservoirs], must be increased significantly. From a review of published literature and their own observations, the authors identified several relevant production technologies that could have a significant effect on tight and shale-gas production in China.
Unconventional Gas Resources
The US Energy Information Administration estimated that China had 0.289×1012 m3 of proved CBM reserves in 2011, although other recent estimates of recoverable reserves are 9.9×1012 m3. Fig. 1 shows most of China’s gas-producing basins. CBM resources are in the north and northeast, the Sichuan basin in the southwest, and the Junggar and Tarim basins in the west. In China, tight gas resources total approximately 12×1012 m3. The Sichuan basin is one of the largest tight gas basins in China and is reported to have approximately 1.8×1012 to 2.25×1012 m3 of natural gas. The Sulige gas field in the Ordos basin is the largest tight gas field in China.
Most of China’s proved shale-gas resources are in the Sichuan, Tarim, and Ordos basins. Estimates of China’s total tight and shale-gas resources range from 36×1012 to 100×1012 m3. Although there was no commercial shale-gas production as of 2012, the Chinese Ministry of Land Resources has set aggressive targets of 6.5×109 m3/a by 2015 and at least 59.5×109 m3/a by 2020.
Management of Hydrogen Sulfide (H2S) Risks
Most of China’s sour-natural-gas fields are in the northeastern and western parts of the Sichuan basin (mostly conventional gas fields). An evaluation of shale-gas potential in the Weiyuan area showed significant shale-gas reserves. Therefore, it is important to determine the likelihood of penetrating sour reservoirs and to manage H2S risks. In general, a rigorous H2S safety-management plan will be needed if tight or shale-gas development will require drilling through or near zones containing H2S.
Water for Hydraulic Fracturing
Hydraulic fracturing is needed to stimulate these reservoirs for production. However, hydraulic fracturing requires a substantial amount of water. It is common for one fracture stage to use more than 300,000 lbm of proppant and 2,000 bbl of water, and each well may have several fracturing stages. The large amount of water needed for hydraulic fracturing may come from streams, lakes, and subsurface reservoirs. Water must be transported to the wellsite by pipeline or trucks and must be treated before use. Treatment may include filtration, removal of divalent ions, or addition of biocides, clay stabilizers, and polymer breakers.
In the arid northern and western regions in China, water for hydraulic fracturing needs to come from subsurface reservoirs and will compete with other water uses, such as farming, coal mining, and power generation. The abundance of coal gasification and coal-to-chemical industries in the Ordos basin further constrains water supplies.
Produced water, whether from the formation or backflow of hydraulic-fracturing fluid, must be treated before disposal. Means of disposal include surface disposal, evaporation ponds, injection into a subsurface reservoir, reuse as hydraulic fluid, and reuse for farming. In Sichuan, evaporation ponds and surface disposal are impractical because of intensive farming. Reuse of produced water for hydraulic fracturing most likely will be the environmentally acceptable option, which will reduce the need of sourcing new water and the need for downhole water disposal.
Pad Drilling and Completion
Drilling and completing multiple horizontal wells from a single pad is common practice in some unconventional gas fields in North America (e.g., Marcellus shale in Pennsylvania). Pad drilling and completion enables multiple wells to share surface facilities, thereby reducing environmental effects. However, pad drilling requires introducing new technologies to Chinese operators and service companies, including the following.
- Surveying and anticollision management
- Geosteering in horizontal wells
- Well manufacturing
Geomechanics
Typically, high-yield-shale reservoirs in North America have a total organic content >2.5%, thermal maturity between 0.4 and 2.0%, high content of brittle material (quartz content approximately 30 to 40%), and high gas content (>2.5 m3/t). The Qiongzhusi and Longmaxi shales in the Sichuan basin are similar to major shale basins in the US: total organic compounds >2% and content of brittle material (quartz and feldspar) >40%. However, China’s shales are buried deeper, are brittle (high brittleness index), and are hard (high Young’s modulus), with a high fracture gradient. Therefore, they require a very high injection pressure and high horsepower to fracture, but once fractured, an extensive and complex fracture network probably will develop. The fracture design will likely favor the use of slickwater as the fracturing fluid, high pump rates, large injected volumes, and low proppant concentration.
Sichuan shale formations are in tectonically complex regions. The shale has natural fractures and the local Earth stress is complex. The high formation stresses make initiating hydraulic fractures difficult. Microseismic data indicate that induced fractures do not extend strictly along the direction of maximum horizontal stress. It also appeared that vertical fractures were not realized during staged fracturing. It is worthwhile to mention that most current commercial hydraulic-fracturing-design software has rather simplistic geomechanics and do not incorporate the effects of natural fractures adequately. Their use in designing fracture treatments for unconventional gas reservoirs must be guided by field calibration.
Nondamaging Fracturing Fluids
Reducing Water Blockage. In low-pressure tight gas reservoirs such as those in the Sulige field, water blockage near the wellbore can cause formation damage. Because of the unique diagenetic history in these reservoirs, the original water saturation is less than the irreducible-water saturation. Water-based fluids were used during drilling and completion operations. Invasion of these fluids into the formation increased the near-wellbore water saturation to a value greater than the irreducible-water saturation, decreasing the gas relative permeability and resulting in a low gas-production rate and temporary formation damage. Water blockage causes a 10-fold reduction in the air permeability, and with the very small pore-throat diameters, initiating water production may require a pressure gradient of several MPa to overcome the capillary pressure.
Coinjecting liquid nitrogen with the aqueous fracturing fluid can reduce water blockage. The nitrogen/water mixture reduces the mixture’s density and lightens the liquid column in the casing to reduce the bottomhole pressure and improve water production. Also, nitrogen reduces the leakoff of fracturing fluid into the formation to reduce water blockage. Severe formation damage from water blockage has been reported in shallower low-pressure tight gas reservoirs in western Sichuan basin.
Naturally Fractured Tight Gas Reservoirs. Tight gas reservoirs, especially those with natural fractures, are prone to damage by fluid and solid invasion into the matrix and natural fractures. Once the formation is damaged, it may be impossible to repair the damage. Consequently, it is important to use nondamaging drilling, completion, and fracturing fluids. To reduce formation damage during drilling, underbalanced drilling with air or foam has been used. Table 6 in the complete paper lists various formation-damage mechanisms and mitigation methods for tight gas reservoirs in the Sichuan basin.
Reducing formation damage caused by completion and fracturing fluids will be key areas of research. Solutions include reducing crosslinker concentration and use of delayed crosslinker, more-effective polymer breaker, clay-control chemicals, and nonaqueous fracturing fluid. It is important to test the compatibility of completion and fracturing -fluids with the formation and to adjust the chemistry to avoid reaction with clay minerals and to avoid forming scales. To avoid polymer damage, it must be determined whether these formations can be fractured by use of slickwater with minimal use of polymer. Learnings from recent North American success with slickwater to fracture tight and shale-gas reservoirs could be relevant to Chinese tight reservoirs.
Multistage Fracturing
The tight gas reservoirs in Songliao, Tarim, and deep Sichuan basins have high temperature and high pressure, making hydraulic fracturing difficult. In these basins, hydraulic fracturing often requires pumping at very high surface pressure for prolonged periods. Surface treating pressure can exceed 69 MPa. Limited availability of high-pressure hydraulic-fracturing equipment is a problem. Therefore, hydraulic-fracturing designs that reduce the formation-breakdown pressure and the required horsepower will be beneficial.
Openhole Fracturing. Openhole completion eliminates the frictional pressure loss caused by perforations. Also, it facilitates greater contact between the wellbore and natural fractures. Therefore, it has potential to reduce the pumping pressure needed to break down the formation and the potential to maximize connectivity between the wellbore and the network of natural fractures.
Multistage/-Zone Fracturing. These fracturing treatments in vertical and horizontal wells are instrumental to develop tight and shale-gas reservoirs in North America. These treatments typically use perforate/plug methods with conventional through-tubing fracturing. Though effective, these treatments require multiple trips and have limited capabilities. While conventional coiled tubing provides many treatment advantages, it can limit treatment rates and its reach is constrained in long-horizontal-section shale wells. New and better ways to perform multistage/-zone fracturing in vertical and horizontal wells will be needed.
Conclusions
Advances in these areas should facilitate successful development of unconventional gas reservoirs in China: managing H2S risks, sourcing and disposing of water for hydraulic fracturing, pad drilling and completion, incorporating geomechanics in well and reservoir modeling, and use of nondamaging fluids and improved multistage-fracturing techniques.
This article, written by Dennis Denney, contains highlights of paper IPTC 17096, “Production-Technology Challenges of Tight and Shale-Gas Production in China,” by Hon Chung Lau, SPE, Shell (China) Projects and Technology, and Meng Yu, SPE, Shell International Exploration and Production, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2013 International Petroleum Technology Conference. Reproduced by permission.