Reinjection Experiment at the Patuha Geothermal Field, West Java, Indonesia: A Field Experience and Numerical Analysis

Abstract

Abstract

The status of the geothermal power capacity is about 13.3 Gigawatts as of January 2016, spread across the world, 25% of which is being produced by vapor-dominated reservoir. It would be more significant to bring in mind the fact that only 5.7% of all the operating geothermal reservoirs are vapor-dominated. Vapor-dominated reservoirs are prized because virtually all high-enthalpy produced-fluid is piped to the power turbines. Hence, the associated expense of injection wells and phase separation systems could be minimized. Vapor-dominated field requires a proper strategic management to be well-developed. The Patuha geothermal field is considered as a vapor-dominated system, located in West java, Indonesia. The site has a significant potential of 210 MWe, while the current installed-capacity is 55 MWe. This study is firstly focused to find the reason behind the poor production of idle wells and high production decline rate, and secondly to propose a suitable solution to the diagnosed-problem in order to enhance the production of the reservoir as well as the proper strategy for the field management in order to deal with the production decline rate.

An extensive field-data analysis, together with a literature review were carried out to diagnose the problem. Furthermore, an equivalent continuum approach was employed to simulate the injection of cold water into fractured super-heated vapor reservoir. Taking advantage of the numerical simulator, the reason of the negative well-head pressure observed during injection was understood. Accordingly, a suitable injectivity test for vapor-dominated reservoir was proposed by means of numerical simulation. The main issue was found to be that the natural recharge is very low to replenish the extracted steam. Consequently, an injection strategy with the main purpose of heat production enhancement was suggested for the available idle-wellbores. This strategy changed the injection status of PPL-01B to PPL-01A which has a proper location in comparison with the location of the former well. Due to the fact that no injection test at PPL-06 was reported, a fully methodical reinjection test for PPL-06 was proposed by means of field data analysis, knowledge of other injection test at other vapor-dominated reservoirs worldwide, and the numerical simulation. This injection plan was considered all the possible reservoir responses.

Based on the analysis on the injection test carried out January 12, 2018 at PPL-06, the reservoir properties and the in-situ permeability were estimated. According a numerical model (using equivalent continuum approach), based on this reservoir characterization, was developed to evaluate the effects of the different injection test in the vapor-reservoir. Firstly, the simulation was performed to study the actual injection test and analyze whether the injection test at PPL-06 could improve the production of PPL-02A and PPL-04 whose production were increased during the injection test. The results revealed that with homogeneous permeability and uniform pressure distribution at the reservoir, the improvement could not be supported by the performed-injection test, since a high permeability greater than 15.5 D is required. By this simulation the production decline of PPL-06 after the injection was understood. The pressure decline trend during the injection was simulated.

Simulation of long-term injection test showed that if the injection at PPL-06 lasts long enough (e.g. in the order of 60 days) the production of PPL-02A could be increased up to 20.75 MWe, while no thermal breakthrough was observed. Note that PPL-06 is currently producing less than 1.5 MWe. It seems that PPL-06 could be converted to a successful injector which would improve the production of the system significantly, however, any injection strategy should be monitored and updated over the time to obtain the optimum injection plan

it is due to the fact that the underground fluid paths are complex, and numerical simulations are associated with simplified assumptions, therefore, the numerical simulations and field analysis have to be modified and updated by the actual test results. The results suggested that deep wells seem to be ideal as injection wells at the Patuha field. Finally, the reinjection strategy was proposed to be employed by the field-owner as the proper management strategy instead of drilling of only more production wells (the current strategy at the Patuha field).

Keyword: Reinjection, Vapor-dominated Reservoir, Injectivity Test, Field Data Analysis, Numerical Simulation, Two-phase Flow, Negative Wellhead Pressure, Hydrothermal modeling