Applicability of TOC and COD values was examined as an indicator of organic contamination levels of subsurface brines from water-soluble natural gas fields. It was found that those brines have unique chemical characteristics as shown bellow, comparing with those seawaters. 1) In TOC measurement, elimination of inorganic carbon by PH lowering made higher reproducibility and lower TOC values than those obtained by conventional methods. It was due to dissolved methane in those brines. 2) In COD measurement, organic compounds in those brines were more easily oxidized than those in seawaters. 3) The TOC values of those brines have strong correlation with the COD values. However, there was no such correlation in regard to seawaters. As mentioned above, it can be concluded that the TOC value is a more appropriate indicator of organic contamination levels of subsurface brines from water-soluble natural gas fields than the COD value.
A well production performance program for a horizontal well is now being developed. This program contains two main parts, inflow performance of reservoir fluids into a horizontal wellbore using three productivity index equations and well outflow performance using an empirical pressure drop correlation. The productivity index equations used in the program are (1) a modified vertical well equation, (2) the Peaceman's equation and (3) the Babu and Odeh's equation. A case study was carried out using the program with reservoir and well data in order to investigate the effects of four parameters on the horizontal well performance; (1) three productivity index equations, (2) horizontal well length, (3) completion tubing size in the horizontal well, and (4) vertical/horizontal permeability ratio (kv/kh) in the reservoir. Conclusions of the case study are as follows; (1) There are little differences among the calculation results of the three PI equations, especially if the horizontal well length is more than 500m for this case. (2) Oil production rate is not proportional to the horizontal well length. The reduced drawdown due to turbulent flow along the wellbore may result in the total production rate reaching a certain critical value as a function of wellbore length. (3) A larger diameter tubing reduces the pressure drop in the horizontal wellbore, which results in more uniform influx profile of reservoir fluids into the horizontal wellbore than a smaller diameter tubing. (4) Vertical/horizontal permeability ratio (kv/kh) has the strongest effect on the well performance among the four parameters. If kv/kh is small, it needs a long horizontal well to produce oil at a high rate.
The oil reservoir, which is one of the main reservoir in the company, has a strong water drive mechanism. With the increase in cumulative production from this reservoir, production water cut rapidly increased in several wells and workover job to conduct plug back on to water bearing zones has been carried out since 1980 to prevent the increase of water production. Although water production could be reduced by this operation, productivity of the producers was harmed when high productive interval was plugged back. In 1989, a well worked over to horizonal well after water bearing interval was plugged back in order to lessen water production without reducing productivity. This study was carried out to investigate the effect of horizonal well and discussion was made on how to incorporate the horizontal well to field scale reservoir simulation. Using numerical reservoir simulator, a well model was constructed and the effect on the critical rate was discussed. And the well pseudo function to the horizonal well generated. It is concluded that the horizontal well conversion was effective to increase the critical rate, Secondly generated well pseudo fuction to the horizontal well was physically valid and this pseudo function shall be implemented to the full scale reservoir simulation model.
In acidizing, it is important to make a suitable plan for the characteristics of the objective reservoir. Particularly, since most reservoir rocks in Japan contain a large amount of feldspar and clay, acidizing needs to be carefully designed considering the prevention of secondary formation damage. To do this, the characteristics of acid reaction on the minerals which constitute reservoir rocks in practical fields should be fully understood. The reaction characteristics of hydrochloric acid on calcite were measured using a rotating disk apparatus. This apparatus was originally designed by D. R. BOOMER and has been generally used to investigate the acid reaction kinetics on minerals. The measurements were made on combinations of acid concentration of 7.5, 15 and 25% and disk rotational speeds of 50, 100, 300 and 500rpm at room temperature and 100°C under a pressure of 7 MPa. The results indicated that the reaction of acid on calcite was diffusion limited, and the effective diffusion coefficient could be obtained from the experiments at room temperature. The final purpose of the study is to develop numerical models of matrix acidizing and fracture acidizing. This rotating disk apparatus was proved to be used to get the information on the acid reaction parameters.
JNOC and Teikoku Oil Co. have studied hydraulic fracturing for a naturally fractured deep volcanic reservoir in Niigata prefecture facing the Japan sea since 1988. First two year of the project was spent for pre-fracturing study. Data obtained from mini-frac, such as step rate test, pump in/flow back test, and pump in/shut in test, were used to design the main fracturing job. Well logging date were used to determine the stress distribution of the formation. In 1990, the main fracturing job was done, succeeded in the improvement of the well production rate about twice as much as that before fracturing, but couldn't be injected the scheduled amount of proppant, because of screen out. The high net pressure and two kinds of closure pressure, which were analyzed from mini-frac, indicate the possible presence of natural fractures. Laboratory experiments have been made to inspect the simulation model. Water is injected to acrylic resin or mortal block, which is put in the true triaxal confining pressure apparatus. Fracturing pressure and surface strain can be measured under various confining pressures. Acoustic emission event detection has also tried to investigate formed fracture geometry.