Mature Fractured Reservoirs Can Be Repurposed for Gas Storage
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This paper discusses studies conducted on two California offshore fields that may be abandoned in the near future. These studies examined the feasibility of repurposing these fields for offshore gas storage by using their reservoir voidage and existing pipeline facilities. These storage sites could offer a significant alternative to the current onshore sites located in highly populated urban areas of the state.
California has been a major oil-producing state since the turn of the 20th century. However, in terms of current natural gas production, California’s indigenous production meets only approximately 10% of state demand.
Gas imports are delivered on interstate pipelines from sources in the southwestern and Rocky Mountain regions of the US and western Canada. The majority of gas delivered to California is consumed directly or pumped into onshore storage fields. California currently has 14 onshore gas-storage fields used to stabilize the supply system. The total current onshore storage capacity is approximately 600 Bcf, with the working capacity being approximately 375 Bcf.
Background and Field Characteristics
A year-long study of issues related to gas-storage fields in California concluded that significant investment would be needed for additional pipelines and gas-compression systems if all onshore storage facilities were to be shut down. While new regulations are in place to reduce the risk of current onshore storage fields, alternatives need to be considered that would be reliable for expanded demand. The study concluded that there is no method for storing electricity that can replace the need for gas to meet the winter peak. (Note that approximately 50% of gas is for nonpower uses.) Uncertainties are also associated with renewable sources as a substitute for gas for generating electricity. The study indicated that Californian consumption of natural gas currently is approximately 6.6 Bcf/D. However, the study points out that there may be a drop in demand for natural gas. Several issues loom on the horizon, one being the increasing liability associated with operating gas-storage fields in urban areas. Another is that, if some or all these onshore sites are decommissioned, insufficient storage capacity exists to address increased use of natural gas in California.
An alternative exists in the form of offshore storage fields. Semidepleted offshore fields present an opportunity for this purpose. Some of these oil-producing operations may be reaching the abandonment stage during the next decade. Total abandonments may result in the removal of offshore platforms and plugging of existing wells, denying Californians further access to nearby offshore reservoirs.
Several offshore Californian fields may not possess suitable reservoir structure or the deliverability rates necessary for offshore gas-storage purposes. These fields produce from the Pliocene sands, where flow rates may not be sufficient for storage purposes. However, a class of prolific reservoirs that produce from the naturally fractured rocks of the Monterey formation exists in federal waters and may meet the operating requirements of gas storage. Foremost among these fields are the Point Arguello and the Point Pedernales fields.
Structurally, both fields consist of independent blocks, and gas storage for any of these fields can be managed in individual fault block compartments according to the demand. Water/oil ratio plots show a classic rise of water/oil ratio indicative of natural water drive, which could also help productivity.
Thanks to natural fracture permeabilities, wells in these reservoirs have demonstrated impressively high deliverability. More than 3 decades of oil, water, and gas production have created significant storage capacities. An important concern is the competency of existing pipelines to the shore; these require further testing and potential retrofitting. The sulfur content of the existing gas in these reservoirs may also be an issue. These pipelines can send low-pressure gas to the compressor facilities on the platforms and then can return low-pressure gas to shore for further compression before delivery to market. Gas treatments must be performed on the platforms.
The Monterey formation is a Middle and Upper Miocene biogenic siliceous rock deposit in California discovered in both onshore and offshore fields. Offshore Monterey reservoirs generally consist of brittle rocks exhibiting the most mature form of the formation. In some places, the total thickness is greater than 2,000 ft. The formation, while primarily consisting of fractured cherts, may include other digenetic phases of silica and also dolomites. The Monterey formation in the featured two fields has been prolific in terms of well productivity.
As shown in Figs. 1 and 2, 3D seismic data confirm, and analysis of well behaviors indicates, a number of isolated fault blocks in each field. Studies show that production blocks are separated from each other and little communication exists among those blocks. Another important observation is the competency of the caprock, indicating preservation of initial pressure from the analysis of initial gas/oil ratio.
Availability of existing multiple pipelines of 8 to 12 in. between the platforms in Point Arguello and the 20-in. line connecting the system to the corresponding onshore treatment facilities could meet the transportation requirements of natural gas from these offshore fields. Similar considerations can be made with regard to the 20-in. pipe to the Lompoc facilities. Confirmation of the operating condition and integrity of these pipeline systems for large-volume gas transport will require further study.
In these two fields, well productivities have been high because of natural fractures and, as a result, a significant amount of reservoir voidage has been generated in each field. From 1987 to late 2014, more than 286 million bbl of oil and more than 928 Bcf of natural gas have been produced from the wells in these fields. Half of these wells have produced individually more than 1 million STB. Some have produced more than 13 million STB over the past 3 decades.
The significant voidage implies a huge potential gas-storage capacity. For illustrative purposes, the authors assumed a maximum storage reservoir pressure of 3,000 psi, a temperature of 200°F, formation-oil volume factor of 1.15 STB, and a formation-gas volume factor of 0.0049 scf. Oil and gas production from 1987 to 2016 translated into a total computed voidage that allowed storage of more than 3 Tcf of gas. This volume far exceeds existing onshore storage capacity and does not carry the inherent exposures created by large urban storage sites. Wells drilled and completed in these fields are compliant with modern safety standards devised after the 1969 Santa Barbara spill.
These two offshore fields are attractive potential storage resources because of their voidage capacity, competent caprock, and high permeabilities from a delivery standpoint. Both fields are compartmentalized, and, as such, storage can start in a few blocks and expand as the need arises.
The authors also reviewed the feasibility of using existing pipelines for transporting the produced crude and gas to onshore treatment facilities and for transmitting and receiving gas. Some of these pipelines require conditioning before the proposed field-conversion process. Gas treatment and compression facilities can be accommodated on the platforms and the pipeline connections can transmit low-pressure gas to the platforms. The produced gas then can be repressurized on existing onshore facilities before being sent to gas-transmission lines.
For a limited time, the complete paper SPE 190134 is free to SPE members.
Mature Fractured Reservoirs Can Be Repurposed for Gas Storage
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