Lower Miocene sandstone reservoir in Rand Dong Field, within Block 15-2 offshore Vietnam, has continuous oil production since 1998. Since oil production had steadily declined from 2002 despites additional developments, full field scale application of hydrocarbon gas enhanced oil recovery （HCG-EOR） started to be investigated from 2008 to maximize oil recovery. After the executions of feasibility studies, pilot test and installations of facilities, the project of HCG-EOR full field scale started in October 2014.

During three years of implementation, the project has been achieving the planned enhanced oil recovery while adjusting the scheme of water alternating gas （WAG） injection. Even though some operational tasks arise such as sand production, countermeasures are investigated and being applied as the appropriate way. For the optimization of WAG injection, the areal monitoring and application of WAG injection pattern have been conducted. Furthermore, the effect of HCG-EOR is divided into the primary, secondary and tertiary effects and quantified respectively for the further optimization plan.

This paper presents the outline of the project before HCG-EOR full field scale application, and the ?ndings through the actual operation of HCG-EOR full field scale including results and evaluations for the further oil recovery.

Enhanced oil recovery（EOR）and improved oil recovery（IOR）technologies have been applied to various oil and gas fields for many years. However, the environment for EOR/IOR has been severe in low oil price and increase of development cost and more effective and economical EOR technology is expected. We focused the application of micro bubble as cutting-edge EOR technology.

Micro bubble technology has been applied to various industries as medical, agriculture and environment fields. And micro bubble has some characteristics in relative to the effect of EOR such as improvement of sweep efficiency and oil density and viscosity.

We have studied about the efficiency of micro bubble technology and its mechanism from laboratory test and flow simulation. We evaluated the improvement of the sweep efficiency, solubility to oil and oil recovery by micro bubble injection from core flooding test which was one of the laboratory tests and it was indicated that the capillary pressure affected the injection area from flow simulation.

We expect micro bubble technology will become useful as new EOR technology from our study.

During planning stage of CO₂ EOR （enhanced oil recovery）, evaluation of remaining oil volume is one of the most important works because of its significant impact on field performance and project economics. However, the evaluation becomes very challenging when the concerned field is a legacy field where historical data are insufficient unless reliable data were later supplemented. We describe how to evaluate CO₂ EOR project for a legacy field with insufficient historical data.

To estimate the remaining oil volume in the field, the following multi-scale data were integrated.

1. SCAL （special core analysis） review

2. SWCTT （single well chemical tracer test）

3. Material balance calculation

Based on the result of remaining oil saturation evaluation, a “type curve” that represents production and injection performance of a target area was established and by superposing the “type curve” as per the pattern development schedule, total field production profile was predicted.

The joint research project was started in 2008 to study new microbial enhanced oil recovery (MEOR) method using microbes which convert oil into methane in reservoir. In advance of starting the project， petroleum geochemistry researches were conducted for INPEX domestic oil fields. Clear signs of bio-degradation caused by microbial reactions were observed in some fields while no signs in nearby other fields. Based on the geochemistry research results， we started the project to implement this method in the domestic depleted oil fields. It was intended to initiate the microbial reactions artificially in the reservoir， by injecting water containing microbes. In general， gas recovery factor is higher than that of oil because of the difference in fluid mobility. Therefore， this method can increase hydrocarbon recovery factor by converting immovable oil to movable methane.

In order to evaluate the potential of this method， microbial cultivation experiments have been conducted under the reservoir condition. The bio-augmentation processes were reproduced in the cultivation bottles by mixing oil and water containing the functional microbe communities. The experimental results showed that the oil conversion reactions can be promoted by optimizing cultivation conditions: porous media reproduced by packing sands， in particular. For detailed evaluation under more actual reservoir condition， further microbe cultivation experiment is ongoing in reservoir rock core. In parallel， a field pilot operation is being planned. For estimating the methane producing rate in a well scale， a simple tank model was constructed to evaluate oil conversion reaction near wellbore. Our results show a positive potential of the new MEOR method that converts residual oil into methane in the depleted oil fields.

In this study, for the purpose of enhanced gas recovery from an methane hydrate （MH） reservoir, we conducted experimental and numerical study for acid injection process as a secondary gas recovery after the depressurization operation. This process is based on the concept of effective utilization of heat generation resulting from mineral dissolution via acid injection for the promotion of in-situ dissociation of MH. For the numerical analysis for the acid injection process, based on the assumption that the charge balance in the injected acidic solution after the contact with the solid matrix becomes neutral due to mineral dissolution into the water phase, we defined two acid components before/after the contact with the solid matrix. In addition, the kinetic parameters and heat of mineral dissolution during the acid injection were obtained through laboratory column tests and DSC analysis, respectively, and introduced into the developed numerical model. Using this numerical model, we conducted a series of numerical analyses for the acid injection process after the depressurization operation. From the calculation results, it was found that a high-temperature zone formed by in-situ heat generation resulting from mineral dissolution extended to the side of the production well to promote MH dissociation. Based on the total amount of methane gas trapped in MH at the initial stage, total gas recovery using depressurization and acid injection was estimated to be approximately 90% and we confirmed that the acid injection process is very effective as a secondary gas recovery targeting the MH reservoir under low-temperature and low-pressure condition after the depressurization operation.

This paper evaluates return permeability of reservoir cores damaged by oil-based-mud （OBM） invasion. A main hole of a highly deviated well in a domestic field suffered from formation damage whereas its vertical pilot hole successfully was showing oil. Both holes were drilled by means of OBM resulting in success from a drilling operation point of view; however, it caused formation damage during completion. Two types of solvents were evaluated to remove contaminants created by mixing of OBM and KCl brine. The candidate remedy fluid was screened through laboratory tests and used in wellbore clean-up operation for the subject well. Consequently, the remedy fluid significantly improved the borehole condition by removing contaminants from the wellbore inside, while we still have remaining damage in the near wellbore region. For further investigation on the near wellbore damage, three types of OBM-invaded cores （i.e. systems of synthetic formation brine, KCl brine, and base oil） were prepared assuming water and oil bearing zones, and they were evaluated from aspects of return permeability under the reservoir conditions. Results showed that OBM-damage was found in oil bearing zone, in particular, whereas a little damage in water bearing zone. It revealed that the magnitude of OBM-invasion was larger in oil bearing zone than in water bearing zone. Effectiveness of the solvent was also investigated for removing the OBM-damage using reservoir cores.

Finally, this paper recommended potential risk-evaluation of formation damages caused by OBM, preparation of remedy fluid, and plans of other remedial options （ex. re-perforation, micro-fracturing, etc.） in case of such damages. All should be done in advance.

We have made continuous efforts to develop a middle mioccene siliceous shale distributed in northern Japan since 2012 with governmental supports. The shale formation is called as Onnagawa formation and is roughly estimated to have an order of hundred million barrels of oil as an original in-place. Although we have successfully recovered a certain amount of oil from Onnagawa shale, cost-effective and efficient production methods to exploit oil from Onnagawa shale are still unclear. In our project, two different stimulation approaches were tried: multi-stage hydraulic fracturing and acidizing.

In the multi-stage hydraulic fracturing job along a newly drilled horizontal well, we met serious issues to occur screenouts at some frac stages. Although the high viscous fluid slug and pre-injection of fine particles were adopted to prevent screenouts, it did not work. From the detail investigation of microseismic event rates as well as pumping pressure responses, the cause of screenouts is interpreted to be in excessive leak-off of fracturing fluid to the natural fracture system.

On the other hand, the acidizing job was a great success using mud acid system applied at an existing deviated-well.

After the job, the oil production increased dramatically from 10 bbl/D to 315 bbl/D and shows 100,000 bbl cumulative production for a year. The production improvement is inferred to come from contributions by stimulating natural fracture system filled with acid soluble minerals.

Based on the interpretation of the two field trials and a preliminary acidizing simulation, a hybrid fracturing concept has come up as a cost-effective approach for Onnagawa shale. In the hybrid fracturing process considered, the acid fracturing stimulate natural fractures developed in the deep field away from wellbore and the proppant fracturing injects the proppant placed in the near-wellbore region to prevent closing of fractures from high drawdown pressure at oil production.

Unconsolidated sands often appear as places of unconventional resources under development, but the mechanisms of phenomena occurring during development, such as sand production and hydraulic fracturing, are not well understood yet. This paper experimentally examined the fracture phenomenon accompanying fracturing. To this end, a method to simulate hydraulic fracturing of unconsolidated strata in laboratory experiment was constructed. In the experiments using a specimen made from a mixture of silica sand and kaolin and a fracturing fluid of machine oil, a linearly extending fracture was formed similarly to the case of hard rocks when the proportion of kaolin was large.

However, when the proportion of kaolin is small, fracture was not formed at all. This phenomenon can be interpreted if the fracture is formed by compressive deformation / failure, which is completely different from conventional theory of hydraulic fracturing. That is, since tensile force is not transmitted in unconsolidated sands, gaps are created by pushing the part where the fluid pressure acts and the grains in the vicinity thereof. Also, in the mixture of silica sand and kaolin, as the proportion of kaolin decreases, the interlocking between the grains improves and compressive deformation / failure hardly occurs, so it is considered difficult to form gaps / fracture in hydraulic fracturing.

ADOC, Abu Dhabi Oil Co., Ltd. （Japan）, has been operating three oil fields offshore Abu Dhabi for more than 45 years. Recently, ADOC developed and commenced production from an oil field located in a Marine Protected Area, an area designated as a Biosphere Reserve by the UNESCO Secretariat. Therefore, special environmental measures were considered for the development of the field. The development plan of the field consisted of; 3D seismic acquisition, subsurface studies, zero discharge operations, dredging of a channel and construction of artificial island and surface facilities. This paper will summarize the various challenges of field development in an environmentally sensitive area, the measures that were implemented and the lessons learnt.

Chemical compositions of rock-forming minerals in the pelitic rock, the siliceous rock, the felsic tuff, and the dolerite intrusions from the Onnagawa Formation in Ayukawa oil and gas field were compiled, based on their SEM-EDS analysis as basic data. They have been used for geochemical simulations to evaluate the precipitation and the dissolution of mineral in the water-rock interaction systems. The analyzed mineral groups were feldspar, zeolite, carbonate mineral and clay mineral. They seem to have relatively wide variation of their chemistry or to be important minerals in the simulation. This examination also revealed that the mineral described as talc in the former article （Okubo, et al.,1996） is pyrophyllite. Therefore its occurrence and chemical composition is also reported.

Mr. Ichizo Omura is the first and most famous petroleum geologist among geologists working in Japanese oil companies at the Taisho to early Showa era. A part of his contribution to petroleum exploration and the spread of petroleum geology in Japan, and outlines of three technical journals which played the same role as Journal of the Japanese Association for Petroleum Technology before its publication are presented in this paper.