Minami-Nagaoka Gas Field Research Articles

Unlocking the Potential of Acid Stimulation in Volcanic Rocks: A Successful Case with Integrated Analysis in Minami-Nagaoka Gas Field, Japan

Summary Acid stimulation of volcanic formations is rarely documented in the literature. A recent study however suggested its potential effectiveness through a comprehensive laboratory/modeling analysis and documented substantial permeability enhancement by dissolution of carbonate-cemented fractures in the near-wellbore area to create wormhole-like high-permeability channels. The study also presented a brief description of successful field execution, although operational details and analysis of results were not presented. This work presents in detail the field case of a multistage acidizing treatment in the Minami-Nagaoka gas field, a volcanic reservoir, and demonstrates the effectiveness of acid stimulation with 10% formic acid for productivity enhancement. The selection of a target well relies on the abundance of cemented fractures along a well. The operational design considers multiple field/well characteristics, such as low permeability; long, perforated intervals; and high-temperature conditions. Effectiveness of acid stimulation is evaluated comprehensively and justified by the integration of real-time stimulation diagnostics using distributed temperature sensing (DTS), real-time surveillance of bottomhole key parameters obtained thanks to coiled-tubing (CT) fiber-optic downhole telemetry, pre-/post-acidizing pressure buildup (PBU) tests, and production logging tool (PLT) surveys. A multistage acidizing operation was executed, after completion of a step-rate test during which a pre-acidizing DTS survey was acquired. Eight stages of 10% formic acid injection and seven stages of degradable particulate diverter placement were pumped, followed by brine displacement and a post-acidizing DTS acquisition. In all the stages, acid injection decreased the bottomhole pressure while the use of diverter increased it (by hundreds of psi), thus indicating success in acid stimulation and diversion, respectively. The stimulation almost doubled the gas flow rate just after the operation, and 10 months after the operation, the gas rate is still 1.5 times higher than before intervening. Pre-/post-acidizing PBU tests suggested a substantial reduction of the skin from 1.50 to −1.91. DTS surveying identified one major and three minor fluid-intake intervals through stimulation/diversion, and integrated analysis with PLTs revealed that the substantial improvement in gas rate was primarily coming from a narrow zone located within the major intake interval, where resistive fractures are abundant. The current case demonstrates the effectiveness of 10% formic acid for the stimulation of rocks with carbonate-cemented fractures, which was also proposed by the former study. It also shows that there is still room for further optimization in the operational design. This paper provides insights on acid stimulation in volcanic rocks and highlights its effectiveness through the analysis of a series of data sets. Readers may obtain knowledge on acidizing design, the evaluation of its effectiveness, and the interpretation of results, with lessons learned through job execution. The study will also serve as a reference to evaluate the potential of acid stimulation for the development of other volcanic reservoirs.

The Use of Formic Acid and Organic Mud Acid for Stimulation of Volcanic Rocks in Minami-Nagaoka Gas Field, Japan

Summary Unlike acid stimulation in sandstone and carbonate formations, acid stimulation of volcanic formations is not well documented in the literature, and its effectiveness and applicability are not well understood. This study aims to evaluate acidizing of volcanic rocks (especially rhyolite) as a well stimulation technique through a comprehensive experimental and modeling investigation using formation samples from a volcanic reservoir, the Minami-Nagaoka gas field in Japan. The experimental study consists of rock characterization, solubility tests, coreflooding tests, and batch reactor tests. The rock samples are investigated with computed tomography (CT) for textural characteristics and with X-ray diffraction (XRD) analysis for lithological characteristics. With these results, candidate acid systems are selected, and their effectiveness in terms of the capability of dissolving volcanic rocks is evaluated through acid solubility tests. Acid coreflooding tests are performed using undamaged plug cores to evaluate permeability responses caused by acid/rock reactions under high-temperature and high-pressure conditions (300°F and 3,000 psi, respectively). Batch reactor tests are conducted to quantify damage due to secondary/tertiary reactions. The mineralogical and textural characteristics of the rock samples let us select formic acid as the preflush acid and a mixture of formic acid and hydrofluoric acid (HF) called organic mud acid (OMA) as the main treatment acid. The composition of OMA was a mixture of 9% formic acid and 1% HF or 10% formic acid and 0.5% HF in this work. Results of the coreflooding tests with the preflush acid indicated permeability enhancement in all the samples and, especially in cores with cemented fractures filled by carbonate minerals, substantial permeability enhancement was observed. On the other hand, cores treated with OMA after the preflush indicated further permeability enhancement in some cases without cemented fractures, whereas other cases showed permeability impairment after the OMA injection. Furthermore, results of the batch reactor tests with formic acid indicated low precipitation risks, whereas those with OMA suggested higher precipitation risks. Detailed analysis on the thin sections and residuals of the batch reactor tests with OMA highlighted the precipitation of unique fluorides, and the precipitation risk was modeled and quantitatively evaluated with geochemical simulations. Although there is more room to investigate the risks of the usage of OMA for the volcanic rocks, the results in this work suggest the use of formic acid as a main treatment acid, as in carbonate acidizing, for wells with abundant cemented fractures in near-wellbore regions. This paper provides insights on acid stimulation in volcanic rocks (especially rhyolite). The results provide a fundamental understanding on the acid/rock reactions and the potential benefits/risks for productivity enhancement of wells in the subject volcanic reservoir.

Transition of tubing and casing materials in Minami-Nagaoka gas field

Transition of tubing and casing materials in Minami-Nagaoka gas field

長岡実証試験サイトにおける二酸化炭素挙動モニタリングへの物理検層の適用

A pilot-scale sequestration of CO2 into an onshore aquifer has been conducted by Research Institute of Innovative Technology for the Earth (RITE) in cooperation with Engineering Advancement Association of Japan (ENAA) . The CO2 injection site is located at Minami-Nagaoka gas field, Nagaoka city, Niigata prefecture, Japan. One injection well (IW-1) and three observation wells (OB-2, OB-3, OB-4) were drilled. CO2 was injected into a thin permeable zone of the reservoir at 20-40 tonnes per day. The CO2 injection started on 7 July 2003, and ended on 11 January 2005 with the total CO2 amount of 10,400 tonnes. The pilot-scale demonstration allowed an improved understanding of the CO2 movement in a porous sandstone reservoir, by conducting time-lapse geophysical well logs at the three observation wells. CO2 breakthrough was identified by induction, sonic, and neutron logs. The sonic P-wave velocity decreased up to 28% at OB-2, and 13% at OB-4. Small effects of CO2 saturation on resistivity resulted in small changes in induction logs when the reservoir was partially saturated.

長岡プロジェクトからみた二酸化炭素 地中貯留技術の現状と課題

Several key questions need to be answered when CO2 geological storage is to be undertaken worldwide. How should CO2 be stored underground? Can trapping be assumed in saline formations and can CO2 be retained for long periods safely in the subsurface? The first Japanese pilot-scale CO2 sequestration project in Nagaoka was undertaken to provide answers to these questions. The injection site is located at the Minami-Nagaoka gas field in Nagaoka City, 200km north of Tokyo. Supercritical CO2 was injected into an onshore saline aquifer at a depth of 1,100m. CO2 was injected at a rate of 20 to 40 tonnes per day over an 18-month period, with a cumulative amount of 10,400 tonnes. A series of monitoring activities, which consisted of time-lapse well logging, crosswell seismic tomography, 3D seismic survey and formation fluid sampling, was carried out successfully to monitor CO2 movement in the sandstone reservoir. This paper presents an overview of the results obtained from both field and laboratory studies to examine the spatial-time distribution of CO2 and various trapping mechanisms in the reservoir. CO2 breakthrough at two of the three observation wells was clearly identified by changes in resistivity, sonic P-wave velocity and neutron porosity from time-lapse well logging. Each velocity difference tomogram obtained by crosswell seismic tomography showed a striking anomaly area around the injection well. As the amount of injected CO2 increased, the low-velocity zone expanded preferentially along the formation up-dip direction during the first two monitoring surveys and less change around the CO2-bearing zone could be confirmed from the following surveys. Unfortunately there was no significant change in 3D seismic results due to CO2 injection. The pilot-scale project demonstrated that CO2 can be injected into a deep saline aquifer without adverse health, safety or environmental effects. The Nagaoka project also provides unique data to develop economically viable, environmentally effective options for reducing carbon emissions in Japan.

プロダクション検層データに基づいたフラクチャリング坑井への追加穿孔の試み

In the Minami Nagaoka gas field located in Niigata Prefecture, Japan, additional perforations were conducted in a hydraulically fractured well to increase the productivity of the well. To our best knowledge, successful additiona lperforation treatments in hydraulically fractured wells have not been reported. A well, Minami Nagaoka MHF-1, was performed six-stages fracturing with limited perforation intervals. Since the production logging data was necessary to perform an efficient perforation job, the production logging was run just before the additional perforations. The additional perforations were conducted at some intervals where gas inflow concentrated to mitigate non-Darcy effects due to the excess gas inflow. Post-job evaluation confirmed that the additional perforations contributed to increase the productivity by a factor of two.

帯水層を利用した二酸化炭素地中貯留実証試験の概要

A pilot-scale CO2 sequestration test into an onshore saline aquifer has been conducted in Nagaoka-City, 200km north of Tokyo under cooperation of Research Institute of Innovative Technology for the Earth (RITE) and Engineering Advancement Association of Japan (ENAA).The aquifer, 1,100 meters in depth and 12 meters in thickness, is in shallower zone of Minami-Nagaoka gas field which is being operated by Teikoku Oil Co., Ltd. (TOC). An injection well and three observation ones were drilled in the site. CO2 of supercritical phase had been injected into a permeable zone in the aquifer with the rate of 20 to 40 tonnes per day. The injection started on July 7, 2003 and ended on January 11, 2005 with total amount of 10,405 tonnes. A series of monitoring method including time-lapse well logging, time-lapse cross-well seismic tomography, bottom-hole pressure/temperature measurement, fluid sampling and microseismicity monitoring have been successfully carried out to grasp the movement of injected CO2 during and after the injection. History-matching simulation had been performed to interpret the monitoring results. Long-term CO2 movement was predicted using the last model of history matching, implying the location and size of the CO2 to remain almost unchanged from those at the end of injection in the test area over a period as long as 1,000 years. The monitoring at the test site will be continued until 2007.

南長岡ガス田周辺の火山岩貯留岩の分布と特性

The distribution pattern of the Green Tuff rock type and facies in the Niigata Sedimentary Basin, with a central focus on the ‘Nagaoka Area’, may have been controlled by rifting and other structural development in the Nanatani Stage. The term “Volcanic Sequence” model is now propounded as a volcanostratigraphic framework for the compound Green Tuff succession which encountered in the Minami Nagaoka Gas Field, based on the comprehensive interpretation and crossed understanding of a large variety of volcanic rock types and facies and their changes in physicality and geochemical composition. The model comprises five volcanic sequences such as the Volcanic Sequence I, II, IIIR, IIIB and IV in ascending order. The Volcanic Sequence I and II consist of non-acidic rocks, and the former characterized with an extremely high resistivity is sharply distinguished from the latter. The acidic Sequence IIIR which has a bimodal relationship with the basaltic Sequence IIIB consists of multi-stacked rhyolitic lava-flows and/or lava-domes. Those are able to be identified based on the composite interpretation of the presumed rock facies together with dipmeter data in each well. The Sequence IV, a thin basaltic flow unit, is limited its distribution to the northern part of the field. Reservoir geological interpretation based on the “Volcanic Sequence” model might be able to explain heterogeneous facies distribution, and different productivities and pressure performances among wells..In the ‘Nagaoka Area’, some of the vintage wells encountered acidic rocks corresponding to the Volcanic Sequence IIIR and the lowermost part of the underlying non-acidic rocks are extremely resistive like the Volcanic Sequence I. In order to predict the distribution of acidic volcanic bodies which remain undiscovered, it is worth trying to re-interpret the vintage wells and seismic data from the aspect of the “Volcanic Sequence”

南長岡ガス田における仕上げの変遷

Although the 21st year could be gone after the Minami Nagaoka gas field started production, this period was also the years which had the problem including the matter described below and have continued planning of that measure, execution, and surveillance. The measure taken by the meantime has just been going to change along with development of the technology which surrounds oil industry. This report describes how tradition of the know-how of the well finish including the work over carried out until now, technology, and skill is performed exactly, and aims at a well service free.

枯渇油・ガス田の有効利用

Teikoku Oil Co., Ltd. operates Minami-Nagaoka gas field, one of the biggest natural gas fields in Japan. Increasing demand of natural gas requires that Teikoku Oil should reinforce its production capability, including number of production wells, in Minami-Nagaoka. From a viewpoint of stable supply of gas resources, importance of natural gas underground storage has been widely recognized. Teikoku Oil established, for the first time in Japan in 1969, the underground storage system in Sekihara gas field, which was a depleted gas reservoir located a few kilometers north to Minami-Nagaoka. Since then, it has played a great role for peak shaving and/or emergency stockpile in Minami-Nagaoka gas production operation. At present, approximately 140 MMNm3 of natural gas is stored with the maximum deliverability of 1.6 MMNm3/day. Some technical issues encountered and solved during the storage operation for these 30 years are discussed in this lecture. In addition, Teikoku Oil notices that its experiences gained through the underground storage of natural gas in Sekihara could be directly applied to underground storage of carbon dioxide. Namely, injecting and storing man-made emissions of carbon dioxide, a greenhouse gas, in underground is considered as one of the possible measures against a global warming problem. Teikoku Oil is a member of a national research project on the underground storage of carbon dioxide led by RITE (Research Institute of Innovated Technology of Earth) and SEC (Safety and Environment Center for Petroleum Development). A pilot test to inject and store 15, 000 tons of carbon dioxide into a water zone is scheduled in and after 2003, for which one injection and two observation wells have already been drilled in Minami-Nagaoka. This noteworthy project is also introduced in this lecture. A key concept common to the both topics above is “utilization of depleted gas/oil reservoirs.”

離散岩体を含む火山岩貯留層における有効浸透率評価

Heterogeneity observed in Minami-Nagaoka gas field is attributed to the presence of discrete rock bodies with a limited extent andimpermeable edges. Obtaining relevant parameters through comprehensive well-test analyses enables us to generate stochastic realizations of discrete rock bodies. For the entire reservoir, however, 40 million cells are required to honor individual geometries, which is too many to be used in flow simulation.To overcome such difficulties, scaleup techniques are often adopted to coarsen detailed systems to scales appropriate for flow simulation. There arises a problem of how to compute effective permeability (keff) at larger scales on the basis of the geological model. Of interest in this paper are to establish a methodology for computing keff appropriate to the reservoir containing discrete rock bodies and to develop keff correlation by conducting sensitivity studies. The resultant correlation equation is verified against naive permeability distributions and its utility is demonstrated through history matching procedures for the actual pressure data from Minami-Nagaoka gas field.

大深度における水圧破砕作業の実施について

A deep well multi stage fracturing project is under going the Minami Nagaoka gas field. Permeability of the target formation is 10-2md order of tight volcanic rocks, deposited deeper than 4, 000m, which creates a high temperature, and high pressure circumstance. In addition, rock facies changes almost foot by foot, and some regions are highly naturally fractured. These characteristics make it difficult to have successful frac operation by a classical approach. Recent study and feed back from the two premature screen out experiences in the Minami Nagaoka gas field suggested that near wellbore tortuosity, which caused multiple fractures to grow, was estimated to be higher than anticipated. In this condition, the net fracturing pressure became increasing due to individual fracture push each other with the fracture width keeping narrow. Increasing net pressure led to the high surface treating pressure, the injection rate would be limited, finally ended in premature screen out.To eliminate this phenomenon, the newly drilled well last year was designed for the high-pressure fracture treatment. Several new design concepts such as extremely overbalanced perforation (EOB), step down test, and proppant slug techniques are planned to observe the degree of near wellbore tortuosity and will be tried to reduce these tortuous paths.

南長岡ガス田における火山岩貯留岩の分布と変質作用

Volcanic rock units in the Minami-Nagaoka gas field are very heterogeneous because of their facies change and intense alteration. ln this study, a geological model of the Minami-Nagaoka gas field is constructed to explain such heterogeneity for optimizing further exploitation. This model emphasizes two aspects of this volcanic reservoir. The first is distribution of primary volcanic body, and the second is secondary alteration processes. Primary volcanic body is identified on the basis of the difference of the primary plagioclase type in phenocrysts. This volcanic rock is divided into 6 zones according to types of primary feldspar in phenocryst; zone 1, oligoclase; zone 2, oligoclase with labradorite and anorthite; zone3, albite; zone 4, oligoclase and andesine; zone 5, labradorite and anorthite; zone 6, albite. It is clarified that the very productive intervals are within the glassy rhyolite of “zone 3” which extends widely in this field. The glassy rhyolite in zone 3 of the southern area is characterized by perlitictexture, and dissolution pores are dominant in such glassy part. In the northern area, such glassy rhyolite underwent intense silicification and dissolution pores are underdeveloped compared with those in the southern area. It is inferred that such heterogeneity of porosity is due to the difference in recrystallization of glass by the influence of the overlying volcanic activity. Dissolution pores are generated by the process of montmorillonitization in which cations such as potassium and sodium leached out. Montmorillonite changes to sericite (illite) when provided with potassium and to chlorite with iron and/or magnesium. In the lower part of zone 3, chlorite occurs as fill of the pores. This means that the iron and magnesium leached from underlying mafic or intermediate rock units deteriorated reservoir quality with chlorite precipitation in the lowermost interval.

南長岡ガス田における断裂分布の予測 応力分布および岩石物性変化からみた断裂の発達様式

In the Minami Nagaoka gas field, north-northeast to south-southwest trending break-out dominate. The trend of maximum horizontal stress is vertically crossing the break-out that means west-northwest to east-southeast. The minimum horizontal stress value estimated from hydraulic fracturing method is slightly smaller than overburden pressure. The maximum horizontal stress is 1.58 times larger than the minimum horizontal stress. The Green Tuff in this field is expected to be in a strike-slip faulting regime, and the estimated differential stress at the depth of 4, 300m is about 490ksc. The Green Tuff exposed to tectonic stress largely exceeding the horizontal stress of a standard overburden pressure state. Therefore the dominant trend of opening fractures is assessed to be controlled by subsurface stress and to be parallel to the maximum horizontal stress.The results of triaxial compression test show that different lithofacies exhibit different stress-strain profile. This implies that the Green Tuff has the inner stress distribution controlled by lithological variation. Therefore, it is highly possible that the difference in deformation behavior, such as brittleness or ductility of the lithofacies in reservoir creates the environment in which remnant stress and local opening fractures are maintained in the Green Tuff reservoir.The combination of the regional tectonic stress and local stress distribution caused by lithofacies's variation is likely to control the distribution of opening fractures and therefore quality of the Green Tuff reservoir.

圧力解析とプロダクション検層を用いた火山岩貯留層の評価

Large hydrocarbon deposits occur in deep-seated volcanic rocks of the Middle Miocene age in Minami-Nagaoka gas field, Japan. As a consequence of repeated volcanisms, rhyolite eruptiots were deposited one after another forming a thick formation with a rapid change of rock facies, each of which exhibits a different degree of natural-fracture development to another.Most of the wells drilled in this field yield heterogeneous behavior during pressure tests, to which several interpretations are possible and no unique reservoir model can be determined through the limited information of pressure transient data. To overcome this difficulty another set of information is desired, and production loggings are conducted during production and shut-in periods. Crossflow through the wellbore is quantitatively detected through production-log analyses ; and, thus, a layered reservoir model appears to be a good idea to describe the subject formation. This paper describes the comprehensive approach that combines pressure-transient analyses and production-log analyses to characterize the thick volcanic formation with severe heterogeneity.A systematic and comprehensive approach presented in this paper is applicable to any thick formation with vertical heterogeneity.

深掘井におけるチュービング挙動 南長岡地区のパッカーカリキュレーション事例

深掘井におけるチュービング挙動 南長岡地区のパッカーカリキュレーション事例

大深度ガス井における１３Ｃｒ鋼チュービングの腐食事例 南長岡ガス田での事例

The depth of the wells at the Minami-Nagaoka Gas Field located in Niigata Prefecture in Japan is over 14, 000ft with up to 355°F of bottom-hole temperatures. The produced gas contains approximately 6% CO2, 2-5ppm H2S, and up to 200ppm chloride ion. During the workover operation on some of the wells, 13Cr tubings were retrieved and investigated from the corrosion stand point. The following conclusions are drawn from the investigation:1. With temperatures above 248°F and the existence of free water, pitting corrosion was observed on the 13Cr tubings of the well producing 6, 700MMSCF with 866days. The depthrange of the corrosion was 300-5, 200ft.2. Severer pitting corrosion was caused by leakage of the annular brine than that without the leakage. The depth-range of the corrosin was expanded to 330-8, 200ft.3. Crevice corrosion was observed at the faces of thread seals. The cause would be the production fluid that penetrates through opening spaces between tubular thread seals near bottom-hole.4. Scratching surface protective films on the tubings by, for example, wirelines caused localized corrosion.

Advanced Casing Design Against Stress Corrosion Cracking of Grade P-110 Casing in Lignosulfonate Mud

Summary Stress corrosion cracking (SCC) of Grade P-110 casing couplings occurred in the hot, deep Minami-Nagaoka gas field. Detailed stress analyses revealed that the dynamic stress changes resulting from temperature caused by operation played a significant role in those failures. From this information, a new casing design was developed.

Altered rhyolites as oil and gas reservoirs in the Minaminagaoka gas field (Niigata Prefecture, Japan)

Subsurface geology and petrography of volcanic rocks of the recently discovered Minaminagaoka gas field show that good reservoirs in the field are altered rhyolites of the mid-Miocene Nanatani Formation. The Nanatani Formation is composed of mudstone, lava and pyroclastic rock with a composition of rhyolite, andesite and basalt. Rhyolites are divided into rhyolites A, B, B', C and D according to petrographical features. Rhyolite A is a hyaloclastite and pumiceous tuff of glassy rhyolite. Rhyolites B, B' and D are aphyric rhyolite lavas characterized by needle-like plagioclase in groundmass. Rhyolite C is perlite lava. Rhyolites B, D and C are suitable for reservoirs, whose pores are not primary, but are secondary macroscopic druses and micropores formed by hydrothermal alteration. Micropores are openings among crystal grains of quartz and albite and are several tens of micrometers in size. Alteration of rhyolitic rocks is of hydrothermal type and is characterized by a smectite (or smectite/chlorite mixed-layer mineral)-sericite-albite-quartz-carbonate mineral assemblage in the upper zone, but clay minerals in the lower zone are chlorite and sericite. Alteration of andesite and basalt is similar to that of rhyolite in general, but in the Kitafukazawa bore hole, andesite and basalt below 4640-m depth have chlorite-laumonite-prehnite-(pumpellyite-epidote) assemblage which is correlated to the assemblage of the higher part of the laumonite subfacies of zeolite facies. Formation temperature of secondary quartz, estimated by the measurement of the filling temperature of fluid inclusions, is ∼ 150°C and is nearly the same as the present temperature at the bottom of the hole. From the geological environment, characteristics of alteration and paleogeotherm, formation of rhyolite reservoirs and migration of oil and gas can be summarized as follows: (1) formation of volcanic piles by submarine volcanism of rhyolite, andesite and basalt, and deposition of mudstone around the piles; (2) generation of hydrocarbon in mudstone along with increasing temperature related to volcanism; and (3) formation of rhyolite reservoirs by hydrothermal alteration, followed by lateral migration of oil and gas into secondary pores.

南長岡ガス田の仕上

南長岡ガス田の仕上