Successes Achieved in Acidizing of Geothermal Wells in Indonesia

Abstract

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.

Chemical stimulation of geothermal reservoirs using retarded acid systems: current developments and potential directions

“…However, in contact with the water in the reservoir, it decomposes to release the acid, so that the dissociation can start. Sandstone acid is an acid system based on organo phosphonic acid that generates hydrofluoric acid in-situ (Pasikki et al 2006). It is likely that the sandstone acid and retarded phosphonic hydrofluoric acid (RPHF) are the same or at least very similar stimulation fluids as RPHF is also created from phosphonic acid and hydrofluoric acid (Barrios et al 2011), sometimes in addition with hydrochloric acid (Alcalá 2012).…”

“…Detailed mechanisms or exact formulations of the two stimulation fluids were not provided in the respective publications. Regardless, the phosphonic acid-based systems have often been applied in geothermal well stimulation in El Salvador (Gomez et al 2009;Barrios et al 2011;Alcalá 2012), Nicaragua (Gomez et al 2009), the Philippines (Barrios et al 2011) and Indonesia (Pasikki et al 2006(Pasikki et al , 2010. Another retarded version of hydrofluoric acid is organic clay acid (OCA) produced by Schlumberger, which consists of an organic acid as well as fluoroboric acid that undergoes hydrolysis in the reservoir to release the hydrofluoric acid (Jaramillo et al 2010).…”

“…As a rule of thumb, the reaction rate halves for a reduction in the temperature of 10 • C . Although the deliberate reduction of the temperature in the context of geothermal applications seems odd, it is a widely used technique found in five different countries to quench the well with cold water to reduce the downhole temperature (Pasikki et al 2006(Pasikki et al , 2010Lim et al 2011;Barrios et al 2011;Akin et al 2015;Eker et al 2017;Goh et al 2020). It is not only applied to slow down the reaction rate but to reduce the corrosiveness of the acids as well which is also temperature dependent (Pasikki et al 2006;Barrios et al 2011).…”

Analisa Perilaku Reservoir Geothermal Menggunakan Metode Pressure, Temperature, Spinner (PTS) dan Gross Permeability Test

“…Energi panas bumi telah menjadi salah satu energi alternatif penting di seluruh dunia selama beberapa tahun terakhir. Energi terbarukan ini telah dimanfaatkan untuk pembangkit listrik khususnya di 24 negara seperti Amerika Serikat, Filipina, Indonesia, Meksiko dan Selandia Baru yang merupakan salah satu negara yang memiliki potensi besar dalam hal kapasitas daya listrik terpasang (Daniswara et al, 2019;Mahajan et al, 2006). Dalam hal pasokan energi primer yang tersedia, energi panasbumi memiliki kapasitas yang sangat kecil dibandingkan dengan energi lain seperti minyak, batubara, dan gas alam yang terdiri 31.7%, 29%, dan 21% dari masing-masing total energi primer pasokan di seluruh dunia.…”