Formation Damage Induced by Fracture Fluids in Coalbed Methane Reservoirs
- Zhixi Chen (U. of New South Wales) | Naseeruddin Khaja | Karen Valencia (University of New south Wales) | Sheikh S. Rahman (U. of New South Wales)
- Document ID
- Society of Petroleum Engineers
- SPE Asia Pacific Oil & Gas Conference and Exhibition, 11-13 September, Adelaide, Australia
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.6.9 Coring, Fishing, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.8.3 Coal Seam Gas, 4.6 Natural Gas, 4.1.2 Separation and Treating
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This paper presents the results of a laboratory study on the effect of different fracture fluid systems on permeability impairment of a typical coalbed methane (CBM) reservoir. These fluid systems include conventional gel fluids (linear and cross-linked gel), gel fluid with surfactant and a viscoelastic fluid system. A series of flow tests on coal plugs were conducted under simulated reservoir conditions to assess permeability reduction due to matrix swelling and cleat plugging by gel fluids. Tests included surface behavior of different fracture fluids and surfactants on coal surface, degrees of matrix swelling and plugging of fractures and cleat systems by fracture fluids.
The results of these tests have shown that permeability impairment induced by matrix swelling is highly irreversible. This irreversible damage can be prevented to a certain extent by conventional practices of adding certain types of salt (such as KCl) into fracture fluids. Both linear gel and cross-linked gel fluids cause a significant reduction (around 70%) in permeability of CBM reservoirs. Addition of KCl along with certain types of surfactants to gel fluid can marginally improve gel clean up. However, the permeability impairment could be as high as 60%. With the use of viscoelastic fluid system, on the other hand, permeability impairment can be as little as 20 to 30%. This means the viscoelastic fluid system has a great potential in reducing permeability impairment which in turn can help rapid dewatering from CBM reservoirs and increase production.
To improve productivity, CBM reservoirs are routinely stimulated by hydraulic fracturing using proppant-laden gelled fluids, which usually contain polymers, surfactants, friction reducers and other chemicals. Fracturing with gelled fluids has the potential to damage reservoir (permeability reduction) by filtrate invasion (Olsen, et al., 2003). There exist, among others, two main mechanisms of formation damage due to hydraulic fracturing treatment in CBM reservoirs: permeability damage due to matrix swelling and cleat plugging.
Coal formations normally contain a certain amount of clay components, such as smectite, illite, kaolinite, calcite, chlorite, etc., which can be affected by an invading incompatible filtrate of water-based fracturing fluids and may lead to coal matrix swelling, and consequently to a relatively large reduction in permeability. This type of sorption/swelling induced permeability damage is highly irreversible (Puri, et al., 1991).
Coal is a dual porosity reservoir rock which has a micro porous matrix and a network of natural fractures known as cleats. Although cleats have very low porosity, they are solely responsible for the permeability of a coal seam. Therefore, coal permeability can be easily damaged due to plugging of cleats by gelled fluids.
To minimize coal permeability damage due to matrix swelling, it is a general practice to add clay stabilizer which is commonly known as surfactants and salts to the base fracture fluid. KCl is a commonly used salt by the industry. The concentration of KCl, however, is formation dependent. In this study, a series of water sensitivity tests were conducted on coal samples to evaluate quantitatively how coal permeability decreases with base fracture fluid and determine optimum KCl concentration for the coal seams.
To minimize coal permeability damage due to plugging of cleats by gelled fluids during hydraulic fracturing, it is possible that by adding certain surfactant to fracture fluids, the recovery of gelled fluids can be improved, therefore, dewatering and gas production can be enhanced. It is well known that surface tension and contact angle have strong effects on capillary pressure generated by fluids in porous media. In CBM reservoirs, the invaded fracture fluids offer great resistance to flow and are difficult to recover during dewatering or production. Surfactants are generally added to improve the surface properties of rocks and lower the surface tension of the fracture fluids and enhance fluid recovery in low-pressure reservoirs. In this study, a number of surfactants were investigated by evaluating their surface chemistry properties and conducting flow test on coal samples.
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