THE SIMULATION OF THE FRACTURED FAULTED RESERVOIR WITH «ECLIPSE»

K. Jiyembayeva, Z. Pikirbekova,

Kazakh National Research Technical University named after K. Satbayev,

Kazakhstan, Almaty, zarifa_91.91@mail.ru

Abstract. In this study the injection of cold water into a fractured geothermal reservoir is considered. The goal of report is to study the distribution of cold water in geological model. A reservoir simulation program (ECLIPSE – Schlumberger) was used to build hydrodynamic model. This simulator is primarily used for the simulation of oil and gas reservoirs (black oil model or compositional), however it had features that allowed for heat related reservoir models to be simulated. The variables investigated included the geothermal gradient, the water injection rate and temperature, rock and water heat capacity.

Geothermal energy plays a great role in people’s life. Humanity started to use geothermal energy from the earliest times. Heat from the earth can be used in many ways as an energy source, from large and complex power stations to small and comparatively simple pumping systems. Geothermal energy can be transformed into electrical energy or directly used as thermal energy in district heating, agriculture, industrial processes. Use of electricity will increase twice by 2050 year.

Water is reinjected back into the reservoir to maintain reservoir pressure and prevent reservoir depletion. Hot geothermal fluids flow through pipes to a power plant for use in generating electricity.

Keywords: Naturally fractured reservoir, modeling on Eclipse software, geothermal energy, geothermal gradient, rock and water heat capacity.

Geothermal energy is a natural heat of the Earth. It is a clean, renewable resource. Geothermal energy is considered a renewable because the heat outgoing from the interior of the Earth is substantially unlimited. It is produced by dividing the radioactive materials in the Earth's core such as uranium and potassium. The geothermal energy is independence from meteorological conditions, which is an important advantage over other types of renewable energies (e.g. solar, tidal, wind, etc.).

Geothermal power plants transform heat into electrical energy. The first such power plant was put into exploitation in 1904 in Italy. Now these stations were built in 72 countries, is leading the United States, the Philippines, Iceland, Kenya, Russia. Geothermal energy systems are most prevalent in regions of elevated heat flow and vigorous deep fluid circulation, such as The Geysers, USA, and Larderello, Italy [1].

An overview of research work was made in France, Nancy in Lorraine University (ENSG) in laboratory of GeoRessources under the direction of professors Yves Geraud and Irina Panfilov.

The method is based on the use of relationships between geothermal manifestations and thermal data. Next parameters were used in this study: rock properties, thermal conductivity, specific heat of rock, temperature gradient, injected fluid (water) temperature (50°C).

In this study fluid flow (water) in fault zone and protolith with different permeability and water injection rates was examined. Simulation includes 2D and 3D models. The aim of project is to find suitable wells locations which allow to support the reservoir pressure, reinject cold water and not to cool geothermal reservoir. Reinjection of produced geothermal fluid is an important component of most geothermal projects. The geothermal reservoirs will cool down due to the reinjection. Therefore, the temperature of production wells may decrease. This problem can be avoided by a proper location of injection wells in order to minimize the cooling of production wells. The distance between injection and production wells appears to be in the range 750-1500 m estimated by a 3D model and 3300 m by a 2D model.

The rock content of reservoir is mostly presented by granite rocks. Granite is a hard crystalline rock of extremely low porosity and permeability [2]. They form by crystallization of certain slow-cooling magma. The main minerals that form granite are quartz, plagioclase feldspars and alkali feldspar, and some amount of biotite, muscovite and/or hornblende. Granite is rich in elements with heat-producing radioactive isotopes (K, Th, U), and is thus commonly associated with temperature anomalies and elevated geothermal gradients within the crust. This feature makes it a suitable geothermal reservoir rock [3].

The thermal waters are mainly hosted within sedimentary reservoirs, consisting of granite with a thickness 1000 m. The Atlantic margin of Morocco extends over nearly 3,000 km from Tangier in the north to Lagouira (La Gouira) in the south. The reservoir is situated at 3000 m below sea level. This basin has a dual structure (rifting and drifting) because of its geological history. A rifting phase, which started in the Late Permian-Triassic, followed by a drifting phase, which initiated in the Early Liassic around 195 Ma

Fractures were generated in reservoirs under the tectonic stress and diagenesis, and these fractures are called natural fractures. Naturally fractured reservoirs are characterized by the presence of matrix and fracture porous media, fluids exist in two interconnected systems. Fractures are highly compressible compared with intact rock, so they may affect overall geomechanical responses significantly, even though they occupy a smaller bulk volume [4].

Natural fractures and faults are the primary pathways for hydrocarbon migration and production in many reservoirs. Unfortunately, they can also act as channels for water breakthrough. The geothermal reservoirs are naturally or artificially fractured for economic feasibility, consisting of the fracture(s) and rock matrix. Because of the different fluid storage and conductivity characteristics of the matrix and fractures, these reservoirs often are called dual-porosity reservoirs [5].

Warren and Root (1963) developed an idealized model to study the characteristic behavior of fractured reservoirs. They used Figure to illustrate their model of a fractured reservoir which is still used in numerical simulation today. A dual porosity model is the most common way of modeling naturally fractured reservoirs. This model based on dividing the reservoir into two main regions: by primary porosity and secondary porosity. The primary porosity represents the matrix, and secondary porosity indicates fracture. Matrix is the plain part of rock. It contains most of hydrocarbons, fluids (water), but it has low permeability and fluid flow cannot occur directly through the matrix to well. Fractures are very high permeable, they are responsible for the fluid flow in the reservoir. The production of fluid to the well is only through the high permeable fractures [6].

Figure 1. Dual porosity model (Warren and Root, 1963)

Nowadays to obtain a more accurate representation of a reservoir, we should not apply a dual porosity model to all reservoir grid blocks. Simulators can manage both dual porosity and single porosity blocks in a single model. For fractured zone, where there is a fracture network influencing to the flow, dual porosity blocks should be used. For areas with no fractures single porosity blocks are used. Fracture zones represent a key element to take into account for predicting the geothermal reservoir life time. The fluid can flow from fracture cells of the Dual Porosity blocks to the Single Porosity blocks and vice versa. Properties of a flow in such a mixed DP/SP reservoir are the distribution of Dual Porosity blocks, properties: permeability and porosity of matrix and fracture.

Water is a decisive component of geothermal systems; it is commonly used in electricity production.

Fluid reinjection is currently used at many geothermal fields in the world. Renjection initially started as a disposal method but has more recently been recognized as an essential and important part of reservoir management. Sustainable geothermal energy use depends on reinjection of produced fluid to enhance energy production and maintain reservoir pressure. So injection in geothermal reservoir is not only an environmental pre-requisite for disposal of the heat depleted fluid, it also helps maintain reservoir pressure.

Reinjection may lead to cooling of production wells. One of the important problems with reinjection is the possibility of an early thermal breakthrough in production wells. Temperature breakthrough and cooling of reservoir continue to be problems associated with injection into the geothermal reservoir. Therefore, the location of injection and production wells must be done properly in order to avoid thermal interference. In this study various aspects of reinjection is studied.

The production temperature depends on injection rate and injection temperature [7].

The aim of reservoir simulation carried out for this area was to build, and find a model that can increase geothermal reservoir life during reinjection of cold water.

Based on the geological data of the reservoir the first step was to build 2D dual porosity model on Eclipse. The reservoir parameters were assigned to the model based on the field measurements: Protolith part 3 km, fault zone 300 m, core zone 0.4 m.

Dual porosity model features in Eclipse 100 simulation:

The number of layers in the Z-direction should be doubled in a dual porosity run of Eclipse,

It is necessary to add NODPPM keyword to introduce the absolute permeability of fractures.

Or modify using effective permeability to absolute permeability in fractures:

PERMX ( fr ) = PERMX ( fr ) × PORO ( fr )

It is necessary to connect wells with fractures

It is necessary to add DPGRID keyword simplify the input of data for dual porosity runs. (Eclipse Technical description, dual porosity model)

Figure 2. Picture of dual porosity in Eclipse

In vertical profile 3 layers were considered. I layer weathered granite with high permeability; II layer and III layers are fractured granite with lower permeability. In order to assign dual porosity behavior to the model, DPNUM and SIGMA keywords were used. It was considered that there are two interacting media, the matrix and the fracture, and the type of flow in the reservoir is mainly fracture flow. In 2D model 2 wells are used. The distance between production and injection wells is 3,3 km. The grid is symmetric containing square cartesian blocks in protolith zone and with increasing size from the middle part to the outer part of the fault area.

Figure 3. Grid construction in fault zone

The main objective at this stage was to determine the required time of flow from injection well into production well with different injection fluid rate at acceptable pressure. For this tracer function was added. Tracers are used in order to qualitatively or quantitatively estimate how fluid flows through the reservoir. The tracer is injected into injection well along with the carrier fluid and detected at a producing well after some period of time. Tracer testing is a standard method of determining mass transport within a geothermal reservoir and can be a valuable tool in the design and management of production and injection operations. The velocity of a moving tracer in a geothermal reservoir is independent of the thermal properties of rock and fluid. Tracer tests can provide information about the flow path and the flow velocity of the geothermal fluids between the injection and production wells [8].

After calculation with different time steps the simulation has been performed for a period of 35 years. After 35 years 100% concentration of tracer will reach the production well through I layer. The injection flow rate is the same as production flow rate.

At next step temperature gradient and thermal conductivity were added to the model. The temperature of the reservoir is varying between 150°C to 195°C from top to the bottom.

One of the main problems in geothermal reservoir is formation of cold water spot, which leads to future cooling of reservoir. Therefore it is important to pay attention for changing of temperature in reservoir during injection of cold water. Different versions of well location were considered with different injection rate. Injection rate also plays great role because if water will be injected with high rate water will reach production well faster, it can cool reservoir quickly, also it influences on reservoir pressure. It was observed that pressure value in fractured layer is greater than in single porosity layer.

Goal is study of cold water propagation in present geological model. To be more precise, it’s necessary to reduce the cold water appearance in fractured, liquid-phase geothermal reservoir.

Two separate simulation runs were carried out: one simulation with permeable core zone and second with lower permeable core zone.

Increased permeability allows fluid to circulate throughout the fractured rock and to transport heat to the surface where electricity can be generated. The main parameters that affect the heat extraction and electricity generation are reservoir porosity, permeability and water production rate. Higher permeability or higher water production rate will be favorable for improving the electricity generation performance.

Fracture permeability was increased to obtain more quickly flow of water through fractured reservoir and cold water occurrence was observed.

Figure 4. The water flow comparison and temperature changes.

Geothermal energy is one of the prospect energy in the future. The geothermal is renewable and clear energy, so it is an important subject to study and to develop the geothermal energy. Geothermal energy enjoys a special position amongst the renewable energy sources because it is available all year round, at any time of the day, and can therefore be used for base load energy, for both heat and power production.

The study focused in geothermal naturally fractured faulted reservoir in Atlantic Coast of Morocco. Hydrodynamic models in 2D and 3D were built. Propagation of cold water in geological model with different rock properties was studied.

One of the important problems with reinjection of cold water is the possibility of an early thermal breakthrough in production wells.

Simulations with different injection zones and different rock properties were tested in 2D and in 3D.

According to the study results, the following conclusions are made:

• The best well location is when injection wells are located in protolith.

• 2 parts (fault and weathering layer)have contribution for the flowing in reservoir

• Permeability of core zone controls the fluid pressure

• To prevent the reservoir from cooling permeability in all layers better to be same, in this case reservoir pressure will not increase, and cold water appearance will present at later moment.

References

1. Bertani, R. Geothermal Power Generation in the World 2005-2010 Update Report // World Geothermal Congress, 2010.

2. M. Souley, F. Homand, S. Pepa, D. Hoxha.- Damage-induced permeability changes in granite: a case example at the URL in Canada Int J Rock Mech Min Sci, 38 (2001).- pp. 297–310

3. Farndon . - The Complete Guide to Rocks & Minerals Joanna Lorenz.- Hermes House, London (2010)

4. Bai,M. - On equivalence of dual-porosity poroelastic parameters.- J.Geophys. Res. 104, 461–10, - 466.

5. Warren, J.E. and Root, P.J. 1963. - The Behavior of Naturally Fractured Reservoirs.- SPE J. 3 (3): 245–255. SPE-426-PA.

6. Aguilera, R. - Naturally Fractured Reservoirs, Penn Well Books. - Tulsa, Oklahoma.- 521 p., 1995.

7. Horne, R.N. - Reservoir engineering aspects of reinjection. Geothermics.- 14 (2–3), 449–457

8. Axelsson, G., and Stefansson, V. - Reinjection and geothermal reservoir management – associated benefits. - 1999 Ljubljana, Slovenia.- 21pp