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Geothermal reservoirs differ significantly from their hydrocarbon counterparts and so do their remedial treatments. Typically these reservoirs are naturally fractured volcanic sandstones with varying mineralogy found over a wide temperature range. Often the zones of low permeability limit the efficient recovery of heat from these reservoirs. The low permeability could either be inherent or induced by damages occurred while drilling and/or production phases. The drilling phase often experience significant fluid losses, which cool the well and near wellbore zone from the high bottom hole static temperature to the moderate levels that make drilling, cementing and completion of these wells possible. The loss of cuttings and drilling mud to permeable formations and/ or fractures is a major source of damage. Other potential damages occur when the well is put on production. As such it is common for these wells to experience a decline in production owing to such formation impairments. Permeability in these zones may be enhanced through methods of well stimulation such as acidizing and high-rate injection. Such practices usually employing mixtures of hydrochloric and hydrofluoric acids in various concentrations, have widely been used worldwide but performance has been somewhat spotty and unpredictable. Also owing to the typical difficult locations of geothermal sites, there have been concerns about the logistical and QHSE aspects while planning conventional treatments. A successful application of a unique acidizing system & other techniques into geothermal wells in Indonesia is presented in this paper. This acid system involves controlled in-situ generation of HF acids based on organo phosphonic acid chemistry. It provides unique advantages in geothermal application in terms of both the enhanced well performance and rightly addressing all the logistics/QHSE issues discussed above. The last treatment with this system resulted in almost 3 folds increase in productivity of well. 1. Introduction Make up steam supply production wells are now being drilled in the Salak geothermal field, a liquid dominated geothermal resource operated by Chevron in Indonesia. Well Awi 8–7, Awi 10–3 and Awi 8–8 are production wells drilled in the 2004 make up steam supply drilling program. After completion, those wells delivered steam at sub commercial steam flow rates at system operating pressures. A comprehensive well stimulation program was planned and executed. The scope of the program included the diagnostic work to identify the causes, stimulation design to determine the most effective stimulation techniques, stimulation execution to carry out the stimulation job efficiently and safely, followed by evaluation to assess the result. Well Awi 8–7 was expected to produce 200 kph of low-gas- content steam. Despite promising indications, the initial steam flow rate from this well was below expectations. A completion test, consisting of pressure-temperature-spinner (PTS) survey, injectivity test, and pressure fall off (PFO) test was conducted to diagnose the problem. These tests were used to characterize the initial state of individual permeable zones to base a stimulation decision on. Injectivity and pressure fall-off (PFO) tests indicated that Awi 8–7 well had low injectivity index (II) and a positive skin. This data supported the fact that the well lost about 94,500 bbls of water-based mud from the drilling process has suggested the presence of the near-wellbore formation damage. Acid stimulation was designed and performed to recover the well. The acid was placed to the target zones via a 2″ coiled tubing. Post-acidizing well test analysis suggested the acid stimulation has successfully improved overall well characteristic. Total II increased from 2.56 to 6.55 kph/psi, permeability-thickness (kh) product increased from 252,000 to 403,000 md-ft, and the skin decreased from +2.2 to -1.2 Flow performance test after the acid job has confirmed a significant improvement of Awi 8–7 deliverability. This test confirmed the increase of maximum discharge pressure from 211 to 297 psig. The production output at stabilized flowing wellhead pressure increased from 51 to 160 kilo pounds per hour (kph) of steam.
Geothermal reservoirs differ significantly from their hydrocarbon counterparts and so do their remedial treatments. Typically these reservoirs are naturally fractured volcanic sandstones with varying mineralogy found over a wide temperature range. Often the zones of low permeability limit the efficient recovery of heat from these reservoirs. The low permeability could either be inherent or induced by damages occurred while drilling and/or production phases. The drilling phase often experience significant fluid losses, which cool the well and near wellbore zone from the high bottom hole static temperature to the moderate levels that make drilling, cementing and completion of these wells possible. The loss of cuttings and drilling mud to permeable formations and/ or fractures is a major source of damage. Other potential damages occur when the well is put on production. As such it is common for these wells to experience a decline in production owing to such formation impairments. Permeability in these zones may be enhanced through methods of well stimulation such as acidizing and high-rate injection. Such practices usually employing mixtures of hydrochloric and hydrofluoric acids in various concentrations, have widely been used worldwide but performance has been somewhat spotty and unpredictable. Also owing to the typical difficult locations of geothermal sites, there have been concerns about the logistical and QHSE aspects while planning conventional treatments. A successful application of a unique acidizing system & other techniques into geothermal wells in Indonesia is presented in this paper. This acid system involves controlled in-situ generation of HF acids based on organo phosphonic acid chemistry. It provides unique advantages in geothermal application in terms of both the enhanced well performance and rightly addressing all the logistics/QHSE issues discussed above. The last treatment with this system resulted in almost 3 folds increase in productivity of well. 1. Introduction Make up steam supply production wells are now being drilled in the Salak geothermal field, a liquid dominated geothermal resource operated by Chevron in Indonesia. Well Awi 8–7, Awi 10–3 and Awi 8–8 are production wells drilled in the 2004 make up steam supply drilling program. After completion, those wells delivered steam at sub commercial steam flow rates at system operating pressures. A comprehensive well stimulation program was planned and executed. The scope of the program included the diagnostic work to identify the causes, stimulation design to determine the most effective stimulation techniques, stimulation execution to carry out the stimulation job efficiently and safely, followed by evaluation to assess the result. Well Awi 8–7 was expected to produce 200 kph of low-gas- content steam. Despite promising indications, the initial steam flow rate from this well was below expectations. A completion test, consisting of pressure-temperature-spinner (PTS) survey, injectivity test, and pressure fall off (PFO) test was conducted to diagnose the problem. These tests were used to characterize the initial state of individual permeable zones to base a stimulation decision on. Injectivity and pressure fall-off (PFO) tests indicated that Awi 8–7 well had low injectivity index (II) and a positive skin. This data supported the fact that the well lost about 94,500 bbls of water-based mud from the drilling process has suggested the presence of the near-wellbore formation damage. Acid stimulation was designed and performed to recover the well. The acid was placed to the target zones via a 2″ coiled tubing. Post-acidizing well test analysis suggested the acid stimulation has successfully improved overall well characteristic. Total II increased from 2.56 to 6.55 kph/psi, permeability-thickness (kh) product increased from 252,000 to 403,000 md-ft, and the skin decreased from +2.2 to -1.2 Flow performance test after the acid job has confirmed a significant improvement of Awi 8–7 deliverability. This test confirmed the increase of maximum discharge pressure from 211 to 297 psig. The production output at stabilized flowing wellhead pressure increased from 51 to 160 kilo pounds per hour (kph) of steam.
Acid stimulation of sandstone reservoirs is viewed by many as a risky enterprise and one that should be undertaken only as a last resort. Much of this reluctance stems from the complex, heterogeneous nature of sandstones(1) and the unpredictability of their response to conventional oilfield acid formulations. Indeed, with an inappropriate acid design, or poor job procedures, even the best candidate well can be damaged, sometimes irreversibly. With the loss of experience and expertise due to industry downsizing, there is some concern that the risk of such failures has increased. This paper discusses the implementation of an acidising methodology with the goal of attaining 100 percent success in acid stimulation treatments. A cornerstone of the strategy involved the development of a personal acidising mentor for every engineer to help them avoid the pitfalls of inappropriate design. The only practical way to achieve this was by the use of computers, programmed with the many rules used by human experts in the design of acid treatments. These rules take into account such variables as mineralogy, temperature, formation fluids, permeability, porosity and tubular metallurgy, amongst others. Integration of these rules, based on an array of algorithms coupled with heuristic principles, outputs a preferred acid design along with recommended acid volumes, pre- and post-flushes, nitrogen ratios for flowback and a customized message board with hints, tips or warnings about the potential treatment. The design advisor also incorporates recommended loadings of corrosion inhibitors, iron control agents, clay control additives, surfactants, solvents, anti-sludges and diverting agents, etc. as and when required. Application of this approach has helped fill the gap caused by the widespread loss of experienced industry personnel and has accelerated the learning process among younger engineers. It has also helped ensure that only appropriate acid treatments are recommended and pumped in any location, worldwide. The paper reviews the results of over 100 acid jobs designed using this methodology. Introduction The science of acidising has its origins over 100 years ago when Herman Frasch of Standard Oil patented the use of hydrochloric acid to stimulate carbonate formations(2). However, it was not until the 1930's that attempts were made to improve production from sandstone reservoirs by injecting mixtures of hydrochloric and hydrofluoric acids. These early treatments were not particularly successful, however, and this relegated these HCl/HF mixtures to only occasional use in those wells that were deemed to have suffered drilling mud damage. It was not until the 1960's that treatments containing hydrofluoric acid again saw widespread use in well remediation. This was largely due to the publication of studies on the chemical interactions of HF with typical sandstone formation minerals, along with guidelines for treatment optimisation. This work eliminated much of the mystery surrounding the use of HF and put its use in practical terms that petroleum engineers could understand(3). However, in spite of this, acidising sandstone formations remained a hit-or-miss enterprise(4). It was fabulously successful in some areas, totally disastrous in others. Indeed, throughout the 1970's and 80's, most production engineers and service company personnel could recite details of some catastrophic acid job that they had either heard about, or actually been involved with.
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