Abnormal High Skin in Coal Seam Gas Reservoirs
BIEBER, Benjamin, RICHARDSON, Rohan
Engineering Honours 2015
Australian School of Petroleum
University of Adelaide
Coal Seam Gas (CSG) or Coal Bed Methane (CBM) is a supplementary energy source and gas supply to conventional oil and gas. Due to the volume of resources, proximity to Asia and increasing global energy demand, Australia has become the second highest producer of CSG worldwide (Hamawand, et al., 2013). Within Australia, the main CSG developments have been in the Surat and Bowen Basins in Queensland which will be the reference fields for this report.
A significant advantage of CSG is that it usually contains a high composition of methane. This is adsorbed on the coal as opposed to existing in the rock pore spaces, which occurs in conventional gas reservoirs. Within the coal structure, natural fractures exist known as cleats, typically containing water. The method of extracting gas from the coal seam, or desorbing gas from the coal, is done by producing water from the cleats, resulting in reservoir pressure reduction or drawdown. This allows gas to desorb from the coal, move through the cleats into the wellbore and to surface. The ability for water and gas to move through the reservoir is known as permeability.
Critical to understanding and optimising CSG extraction is determining the productivity or flow rate when drilling the well from various parameters, such as permeability and skin. Skin refers to a dimensionless factor used to describe the degree of damage to a formation or influences on a formation that will impede well productivity. A positive skin factor indicates wellbore damage and a negative skin factor indicates enhanced productivity. This project will focus on positive skin in CSG reservoirs, the accuracy of calculation and causes.A method for determining skin values and permeability can be deduced from pressure transient tests. The most common forms of pressure transient tests used in the CSG industry are obtained from drill stem tests (DST) and more recently wireline dual packer tests or wireline formation tests (WFT).
DSTs and wireline dual packer tests involve the process of running isolation packers into the wellbore spanning the interval of interest. The packers are then inflated to isolate the interval from the remaining open hole section. A differential pressure is applied allowing formation fluid to flow into the tool where the pressure response is subsequently monitored. Both forms of testing allow for the isolation of specific zones of a formation in order to estimate formation properties, including near wellbore formation damage, which are used to calculate the well inflow potential.
When employing the above pressure transient test methods in permeable coal seams, high skin values have been observed. Oil and Gas companies generally ignore the high skin values mainly due to the source of high skin being uncertain or in some cases unknown. However, it is important to understand why we observe these values and more importantly, if the estimated permeability and production relating to the skin is reliable.
The purpose of this study was to determine the validity of the skin values deduced from pressure transient test analysis and research theories or mechanisms contributing to high skin in coal seam gas reservoirs. Determination of skin calculation was further assessed against conventional research to further validate its accuracy and recommendations provided to minimize near wellbore damage.
It was identified from the literature review that the significant causes leading to increased near wellbore damage were; filtrate invasion and solids plugging, drilling fluid/reservoir compatibility, stress induced damage from the drill bit and stress dependent permeability effect. In most cases, mitigating solutions were discussed, providing a reference for industry when high skin is encountered.
In terms of test method and calculation accuracy, high skin could be partially explained where high drawdown occurred and two phase flow observed. This is more prevalent when performing DSTs on high permeable reservoirs, however should be monitored when performing WFTs.