Fracture Stimulation in the First Joint-Appraisal Shale-Gas Project in China

This paper discusses fracture stimulation in the first joint-venture shale-gas project with foreign companies in China. The block is in Sichuan province, and the target zone is Longmaxi hot shale, a Silurian formation. Its matrix permeability is extremely low (100 to 300 nanodarcies), but it is rich in natural fractures. Hydraulic fracturing has been shown to be critical to enhancing production. A collaborative approach was applied to the completion of the shale gas wells during appraisal.

Introduction

Most of China’s proved shale-gas resources are in the Sichuan, Tarim, and Ordos basins. China’s technically recoverable shale-gas resources are estimated at 1,275 Tcf. The operator jointly appraised a shale-gas block in Sichuan with PetroChina. The project team needed to make several determinations in the early phase of the project, such as whether there is sufficient gas in the shale formation and whether the gas can be extracted at a high-enough (commercial) rate. From the production technologists’ perspective, the technical objectives of the early wells included the following:

Test the ability of the Longmaxi hot-shale interval to be fractured under high formation pressure.

• Establish baseline fracture designs for the Longmaxi hot-shale interval, at greater than 3500-m true vertical depth (TVD), and systematically evaluate the design parameters.
• Establish an effective multidisciplinary workflow.
• Gather fracture-stimulation-performance data and subsurface-pressure information.
• Determine the productivity of the gas from the appraisal wells to prove the presence of a shale-gas play.

The joint-appraisal phase began in November 2010. While planning the drilling of deep wells to evaluate the Longmaxi formation, data were obtained from several shallow wells at a location close to the surface. Extensive laboratory tests were conducted to evaluate the geochemical properties of the rock. In addition, some rock-mechanics properties were measured. The drilling of the first well (Well A) began in December 2010, and hydraulic fracturing began once the well reached designed depth. To date, five wells have been drilled; two are vertical, and three are horizontal.
A team was established to design and execute the hydraulic-fracture stimulations and production testing of the stimulated wells to reveal the shale-gas-production potential. The team has worked with various disciplines to establish an integrated workflow incorporating shale-gas best practices, especially those from North America. However, one of the key aspects of shale-gas development is that every shale reservoir is different. This means that unique solutions must be crafted on the basis of the specific challenges of the reservoir, such as

• High wellhead treating pressure caused by high formation stress and pore pressure.
• Deep wells (deeper than 3500-m TVD on average).
• The service industry in Sichuan is different from that in other areas of the world in that local service companies provide most stimulation services. The project team had to work with both local and international companies to set up the service infrastructure.
• Fracture geometry (diagnostic technology had to be applied to detect the geometry of the created fracture in order to evaluate the effectiveness of the stimulation).

Hydraulic-Fracture-Stimulation Conceptual Procedures

The project team engaged with internal expertise involved in North American shale-gas operations and industry experts to plan the stimulation strategy. On the basis of the existing information from the shallow cores and the general understanding of the geological and geomechanical setting of the play, conceptual stimulation design and procedures were agreed upon for planning purposes.

Vertical Wells. Vertical wells served important exploratory purposes. A full suite of data-gathering techniques for subsurface evaluation needed to be conducted. Once target zones were identified, the wells were hydraulically fractured to test whether hydraulic-fracture stimulation would enhance production and provide information for further evaluation of well productivity in horizontal wells. The main objective in fracturing the vertical wells was to test the ability of the rock to be fractured to develop the operational windows for further horizontal-well-stimulation design and execution (Fig. 1). With limited information available at the time of planning hydraulic fracturing while the vertical wells were being drilled, a conceptual design was put in place by integrating best practices from existing assets and experience obtained by the industry at large.

Fracturing Design Concept.

• Low-viscosity water [usually called slickwater (SW)] is the main fracturing fluid, with a low concentration of proppant (1 to 2 lbm/gal).
• Pumping procedure: High pump rate (40 to 60 bbl/min) at the highest possible treatment pressure.
• Perforation strategy: limited-entry design flexibility.
• Large proppant volume per unit thickness (1.5 to 2 t/m).
• Liquid volume: 1000 to 1500 m³.

Target.

• If the fracture geometry is of the network type, try to activate the natural-fracture network.
• If the fracture is planar, try to create large fracture half-length (150 to 450 m).
• Maintain post-closure conductivity: no proppant flowback.

Fracturing Operational Procedure.

• Plug, perforate, fracture.
• SW plus low-concentration gel.
• 100-mesh sand and 40/70 ceramic proppant.
• Mill plugs and initiate flowback.

Horizontal Wells. Once a target zone is identified through a successful stimulation of vertical wells, a horizontal well is drilled along the target zone. Then, multiple stages of hydraulic fractures are placed in the horizontal section to evaluate the productivity and economical viability of the shale-gas play. Depending on the understanding of regional and local stress conditions, the well trajectory needs to be planned so that the fractures will grow in a favorable pattern.

Fracture-Design Concept and Fracturing Procedure.

• Design stage spacing on the basis of core and log data.
• Coiled-tubing sand jetting for the first stage, plug/perforate/fracture for the remaining stages.
• High pump rate (greater than 70 bbl/min).
• Hybrid fluid (SW plus low-concentration gel).
• Limited-entry perforation design.
• Proppant quantity of 80 to 120 t per stage.
• Liquid volume: 1000 to 1500 m³ per stage.

Hydraulic-Fracture Designs: Methods and Parameters

As the wells are drilled, cores and logs are acquired; real hydraulic-fracture designs are then conducted by the integrated team. A series of studies are performed to obtain key information related to the identification of target zones and to define key parameters. Methods used in defining some of the parameters of the fracture stimulation design are described in the following:

• Target-zone selection: Log and core-test results. Longmaxi rock properties are most favorable at the base of the hot-shale package.
• Rock mechanics: derived from log data. The log is calibrated with laboratory-test results, such as brittleness and completion-efficiency studies.
• Stress: log-derived, calibrated with core-test-derived correlations and diagnostic-fracture-injection-test (DFIT) data.
• Reservoir pressure: pore-pressure prediction and DFIT data.
• Fluid selection: SW, linear gel, crosslinked gel (database and laboratory tests). Fluid type is dependent upon the rock properties and related to brittleness and hardness. Fluid type varies from hybrid to linear gel to SW to create dendritic fractures. From the rock and geomechanics tests thus far, the Longmaxi shale is more likely to be brittle rock. Therefore, the fluid system is mostly SW with limited linear gel to carry higher concentrations of proppant.
• Proppant: Closure-stress prediction, instantaneous shut-in pressure, and local stress study (DFIT verification). On the basis of the stress conditions, higher-strength ceramic proppant was selected to ensure that fracture conductivity would not restrict the gas flow.
• Fracture-treatment design: Various hydraulic-fracture-design software packages have been used as design tools and for comparison in gaining better understanding of formation and engineering parameters.
• Calibration of fracture-design models: Tracer technology, production logging, and microseismic measures are some methods used to provide fracture-geometry information for data analysis. This is combined with other information such as DFIT data to improve fracture designs.

Horizontal-Well Stimulation. Vertical Wells A and B produced gas to the surface, demonstrating the presence of a shale-gas play. To evaluate the reservoir potential, the team also had to establish potential for commercial flows through horizontal wells. This was performed by the following means:

• Long-term production (greater than 6 months) is typically required for decline analysis to estimate well estimated ultimate recovery (EUR).
• Horizontal wells would increase the probability of encountering natural-fracture systems.
• North American shale-gas development is based on horizontal-well development, where drilling and completions are optimized with successive wells.
• However, the horizontal-section length is typically limited in early wells and increases with experience in the play and overcoming technical challenges. The resulting EUR of the well is sensitive to horizontal length.

For the succeeding wells, the concept has been changed to drilling a pilot vertical well to define the target zones and then sidetrack to drilling a horizontal section covering the targeted zones.
Optimized Perforations (Horizontal Wells). Hydraulic-fracturing design is an iterative process. Once the well is drilled and log information is obtained, the team will go through a process to select the target zones and determine the stages to realize the production-testing objectives. For each selected horizontal section, the number of needed perforation clusters will be determined, along with how to place them in the selected sections to maximize stimulation effectiveness. A perforation-optimization process is part of the iteration process.

Cooperation. Integration and cooperation within the project team and among the parties involved in all phases of the operation are key to the success of the shale-gas project. The project team had engaged all service partners throughout the process to ensure safety, quality, and efficiency. In the meantime, all parties were encouraged to make suggestions for improvement. The influence of North American expertise was also an important part of the team’s cooperative success. In order to optimize data access for people not on the wellsite to provide real-time support, data transmittal through a Wi-Fi network was established.

Key Lessons and Future Work

• The Longmaxi formation has the potential of producing gas with proper well configuration and stimulation.
• The focus will be on increasing EUR per well, reducing costs and land use, and managing water.
• Simulation work is in progress that will combine rock-mechanics-test results to generate correlations to calculate rock properties.
• Stimulation-quality study results will be compared to have a better understanding of reservoir properties.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 16762, “Successful Fracture Stimulation in the First Joint-Appraisal Shale-Gas Project in China,” by Liang Jin, Changlong Zhu, Yong Ouyang, Ming Zhou, Qingguang Qu, Fei Li, Roger Yuan, Chris Wu, Zhiyi Zhang, Yan Wang, and Fa Dwan, Shell China Exploration & Production; Sanjay Vitthal, Shell Canada; and Shiqian Wang, PetroChina Southwest, 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.