Recent technological advancements, such as the use of horizontal wells combined with hydraulic fracturing, has led to economic development of ultra-low permeability (unconventional) hydrocarbon reservoirs. Optimization of unconventional reservoir development using horizontal wells, however, requires quantification of critical reservoir properties affecting the flow of hydrocarbons to the well, such as permeability and porosity. There are a variety of laboratory-based (rock) core analysis techniques that have been developed specifically for unconventional reservoirs, each with their own advantages and limitations. However, none of these techniques properly reproduce the conditions under which horizontal wells produce in the field.
“Hydraulically-fractured horizontal wells producing from unconventional reservoirs initiate a pressure transient that propagates out into the reservoir from the hydraulic fracture over time, like waves emanating from a rock thrown into a pond”, says Dr. Christopher R. Clarkson from the University of Calgary. The production data from a well during this transient flow period can be analyzed using an engineering method called rate-transient analysis (RTA) to extract key information, such as hydraulic fracture properties. Models used for RTA, however, commonly require estimates of reservoir permeability and porosity, often determined in the laboratory.
“Our goal is to simulate in the laboratory how a well produces in the field, and apply the same RTA models to core tests. This allows for a more realistic and representative analysis.” Clarkson demonstrated several years ago (Clarkson et al., 2012; Clarkson and Qanbari, 2013) that this was theoretically possible using reservoir simulation, but the method was not tried in the laboratory until now.
With the aid of his laboratory team, consisting of Ms. Atena Vahedian, Dr. Amin Ghanizadeh, and Mr. Chengyao (Charles) Song, a new experimental apparatus was constructed and procedure developed. A proof-of-concept of the RTA core analysis method, using this new equipment and procedure, was published in the research journal, Fuel (Clarkson et al., 2019).
In brief, using a low-permeability sample from Western Canada, methane gas was first injected into the core plug and allowed to stabilize. The gas was then allowed to produce from one end of the sample at approximately constant flowing pressure, and the gas rate produced over time measured. The gas rates were then analyzed using RTA techniques and models, analogously to what is done with well production data in the field.
“The results, while predicted using simulation several years ago, were better than expected, thanks to my research team”, remarks Clarkson. The proof-of-concept test demonstrated the following advantages of the new method: 1) the same flow regimes (patterns of flow during production) observed with field data were observed during the core test; 2) two permeability estimates and a pore volume (and hence porosity) were obtained from RTA of the data, providing an important redundancy for permeability estimation not typically available with other techniques; 3) the permeability and porosity estimates, later measured as a function of pore pressure and effective stress, were repeatable and consistent with independent methods; and, 4) the results were obtained in a fraction (e.g. 10%) of the time of routine unsteady-state core analysis techniques, such as pulse-decay.
In summary, Professor Christopher R. Clarkson and his research team successfully developed a novel core analysis technique that effectively reproduces the conditions that wells experience in the field and can be analyzed using the same methods. Altogether, the University of Calgary study will advance laboratory-based methods for permeability and porosity estimation in unconventional hydrocarbon reservoirs.
Clarkson, C., Vahedian, A., Ghanizadeh, A., & Song, C. (2019). A new low-permeability reservoir core analysis method based on rate-transient analysis theory. Fuel, 235, 1530-1543Go To Fuel
Clarkson, C.R., Nobakht, M., Kaviani, D. and Kantzas, A., 2012. Use of pressure- and rate-transient techniques for analyzing core permeability tests for unconventional reservoirs. Paper SPE 154815 presented at the SPE Americas Unconventional Resources Conference held in Pittsburgh, Pennsylvania, 5-7 June.
Clarkson, C.R., and Qanbari, F., 2013. Use of pressure- and rate-transient techniques for analyzing core permeability tests for unconventional reservoirs: Part 2. Paper SPE 167167 presented at the SPE Unconventional Resources Conference-Canada held in Calgary, Alberta, 5-7 November.