Date(s) - 08/11/2020
11:30 am - 1:00 pm
3D SEISMIC ANALYSIS TO EVALUATE AGI FEASIBILITY OF CARBONATE RESERVOIRS
Speaker: Jasha Cultreri
Authors: Alberto A. Gutierrez, RG ; Jasha Cultreri; Lou Mazzullo, RG
The evaluation of the potential suitability of geologic reservoirs for the permanent disposal of H2S and CO2 requires many of the same methods used in exploration of potential hydrocarbon reservoirs. These tools include traditional geologic methods such as basin, structural, stratigraphic analyses using geophysical logs, production and available well data. The irony of prospecting for acid gas injection (AGI) reservoirs is that while a reservoir with a minimum of penetrations (actual or potential) is most desirable, this condition reduces the data density available to make accurate predictions of reservoir conditions. For these reasons, in areas where reservoirs have not had extensive exploration or penetration, other tools must be used to evaluate the suitability of the potential injection zone. Especially when suitable targets are found at significant depths exceeding 3500 meters, it is critical to reduce the risk of encountering a zone which will not be suitable due to the high cost of drilling and testing wells to these depths.
In the Permian Basin of West Texas and SE New Mexico, good potential AGI reservoirs exist in the Siluro-Devonian and Ordovician section which is comprised of the Devonian, Fusselman and Montoya Formations. These are carbonate reservoirs which were subaerially exposed due to sea level fluctuations resulting in the creation of dissolution features which significantly improve the permeability and porosity of these formations. However, because these events were variable through time and space and the rocks have been subjected to millions of years of structural and hydrothermal alteration, the areas of optimal porosity and permeability have unpredictable and variable boundaries with portions of the formations which have much lower porosity and permeability making them poor AGI candidates. The occurrence of these Karst features and secondary porosity enhancement and dolomitization can be observed in some geophysical logs where they are available but geophysical and geological analysis using seismic data provides an excellent option for evaluating these carbonate reservoirs for AGI potential.
Due to the contrasts in density and corresponding effects on the reflection and refraction of induced seismic waves, these zones show up as anomalously low velocity zones interbedded with other zones of contrasting seismic velocity characteristics. Through the use of multiple vertical slices of 3D data in combination with synthetic seismograms constructed using control wells, the structural and stratigraphic variations which can be used as surrogates for improved porosity and permeability can be mapped in strategic directions within the study areas. In addition, the use of time slices of the 3D seismic data allow for the evaluation of the areal extent of reservoir zones of various potential feasibility for AGI. These analyses will significantly reduce drilling risks and potential economic impacts of drilling a well in a zone which will not provide sufficient permeability to allow injection of treated acid gas (TAG) at desired rates within acceptable and permit-controlled maximum surface injection pressures (MAOP). In addition, the evaluation and better mapping of porosity within a potential AGI reservoir unit will allow for the more accurate prediction of the total capacity of the reservoir.
This paper presents a brief review of the technology, logistics and costs of obtaining 3D seismic data and a detailed examination of its utility in the evaluation of carbonate reservoirs for AGI. The actual use of 3D seismic to evaluate potential AGI reservoirs is evaluated through the examination of two case studies from the Permian Basin where Silurian-Ordovician carbonate reservoirs were evaluated using this tool. One area was evaluated using available seismic data which was provided by an adjacent operator and the second area was evaluated after conducting an 18 square mile survey designed by Geolex, Inc. and shot by Dawson Geophysical. The first of the two locations was tested very successfully with an AGI having been completed in January 2017 which has been operating continuously since that time averaging about 142,000 m3/day (5,000,000 ft3/day) of TAG injected at an average pressure of 10.34 MPa (1500 psig). The second area was tested by a well in 2018. It is now taking about 17,000 bpd at 130 psig surface pressure.
Sophisticated and costly tools such as 3D seismic in order to find the best potential deep AGI targets are being used increasingly in response to the need to develop AGI disposal zones which will serve the needs of increasing sour gas production in the Permian Basin. This is due to the extensive development of zones that are productive of hydrocarbons and the need to go to increasingly deeper and more expensive targets in areas of limited well penetrations of potential AGI reservoirs.
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