Through-ESP Stimulation: A Cost-Effective Alternative
- Karen Bybee (JPT Assistant Technology Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- June 2008
- Document Type
- Journal Paper
- 69 - 71
- 2008. Society of Petroleum Engineers
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- 158 since 2007
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This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 112315, "Through-Pump Stimulation: A Cost- Effective Alternative," by K.I. Ojukwu, SPE, Petroleum Development Oman, prepared for the 2008 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 13-15 February. The paper has not been peer reviewed.
A major disadvantage of electrical submersible pumps (ESPs) is that they block access to well re-entry for surveillance and through-tubing intervention. One case of the latter is acid stimulation. In Petroleum Development Oman (PDO), acid jobs have been carried out as part of work-overs to retrieve the pumps first. The cost implication can be large, ranging from USD 250,000 to 400,000. Two deployment techniques were adopted in PDO recently, bullheading acid into the reservoir through an ESP and spotting acid through coiled tubing (CT) into the reservoir; by so doing, 80 to 93% of comparative cost was saved. The techniques involved the use of abrasion-resistant ESPs for vertical wells and an ESP bypass (Y-tool) for CT deployment in horizontal wells.
The X field in Oman is a sour, low-pressure carbonate formation 600 m deep and under natural depletion since the early 1970s. The field suffered a sharp pressure decline early, leading to installation of gas lift in all wells. By the mid-1990s, the remaining producers were converted to ESP wells because of gas-supply constraints. Gross production increased by more than 200%, and water cut increased from 50 to 80% at that time. A total of 18 wells are completed in this carbonate formation, which includes three cased horizontal and two openhole observation wells. By 2006, only four of these wells were producing, with an 80% average water cut. X-field wells are characterized by high productivity indices (PIs) of 1 to 4 m3/(kPa.d), mainly a result of an active fracture network and faulting. Almost all matrix-completed wells did not produce, causing all production to be from the fracture and fault network.
Several matrix wells had been treated with acid after drilling and before production but still did not produce sustainably. To obtain the best results from acid treatments, spent acid should be removed from the well soon after the treatment, which usually is not a problem in wells that produce naturally. However, in low-pressure wells completed with pumps such as ESPs, post-stimulation acid pumpout is hardly possible and has not been a standard practice. This game change was necessary to exploit matrix production in the X field and would mean developing new ways to deploy acid effectively at a reasonable cost and yet with capability to pump out spent acid in time to avoid further reaction or damage (within 1 to 2 hours).
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