The Research of Gas Flooding of Horizontal Well with SRV in Tight Oil Reservoir

The characteristics of tight oil reservoir are low porosity and ultra-low permeability, thus stimulated reservoir volume (SRV) should be conducted whether applying the mode of vertical wells or horizontal wells production. Tight oil reservoir is mostly developed by natural depletion or water flooding recently, but the problems are existed, including low recovery factor with natural depletion and the difficulty of water injection. To further improve the development effect of tight oil reservoir, CO2 flooding is proposed. Based on changing tight oil reservoir in Ordos Basin, an oil sample of typical block is selected. The PVT experiments are conducted. The compositional numerical model of five-spot pattern is established with a horizontal well in the middle and 4 vertical wells on the edge. Based on the model, several CO2 flooding scenarios of horizontal well with different completion measures are studied. Furthermore, parameters such as the formation pressure, production rate, shut-in gas-oil ratio and total gas injection volume are optimized. The results of this study show that the recovery factor of horizontal well with SRV is higher than those of horizontal well and conventional fractured horizontal well. The study gives new ideas of CO2 flooding with volume fractured horizontal well for the Ordos Basin tight oil reservoir. It can be helpful for rapid and effective development of tight oil reservoirs in Ordos Basin. The Research of Gas Flooding of Horizontal Well with SRV in Tight Oil Reservoir


Introduction
In recent years, it is become a hot spot to develop unconventional reservoirs, such as tight oil, tight gas and shale gas [1,2]. Tight oil is a typical unconventional resource, which has the characteristics of good fluid properties and poor reservoir properties. The permeability and porosity of tight oil reservoir are general less than 1×10 3 μm 2 and 10%, respectively [3]. Currently, the unconventional reservoirs are usually developed by horizontal wells, especially the segmented multi-cluster fractured horizontal wells, which have been widely used worldwide [4]. The simulated reservoir volume (SRV) can be formed around the horizontal well after segmented multi-cluster fracturing [5,6]. The technology of SRV is to achieve the important goal of increasing the contact area between matrix and fractures or fracture network as far as possible during the development of unconventional reservoirs [7,8]. The study and application of developing tight oil and shale oil reservoirs mostly focus on the natural depletion [9]. But some studies have shown that CO 2 flooding is an effective approach of enhanced oil recovery (EOR) in tight reservoirs [6,10,11]. Based on the real case of Changqing tight oil reservoir in Ordos Basin, both PVT experiments and minimum miscible pressure (MMP) experiments were conducted for the crude oil sample of typical block. The compositional numerical model of typical five-spot well group is built, which is used to study the water flooding, CO 2 flooding and Water-Alternate-Gas (WAG) of CO 2 flooding. Different completion measures of horizontal well are analyzed. Furthermore, parameters such as the formation pressure, production rate, shut-in gas-oil ratio and total gas injection volume are analyzed. "Gas Flooding, " also known as miscible flooding, is one of the leading enhanced oil recovery (EOR) technologies employed for recovering oil that was formerly referred to as either stranded or trapped. Gas flooding is an "enhanced oil recovery" application for injecting miscible (and immiscible) gases into an oil reservoir for increasing oil production.
Gas flooding typically includes CO 2 , natural gas or nitrogen as the gas that is injected. Gas flooding tasked place as either a miscible flood or an immiscible flood. Miscible means that the gas that is injected "mixes" with the oil, thereby reducing viscosity and interfacial tension of the oil and rock. Miscible gas flooding also increases oil "swelling" and localized pressure or drive within the reservoir. "Immiscible" flooding means that the gas that is injected into the reservoir does not mix or go into solution. Therefore the purpose of the immiscible flood is to provide the energy or drive by increased pressure. Immiscible flooding does not produce as much oil as miscible gs flooding, however there are certain applications and reservoirs wherein immiscible flooding is well-suited.

PVT and MMP experiments
A crude oil sample is obtained in the tight oil reservoir. The PVT experiments are also conducted, and PR3 equation of state (EOS) is applied in PVT regression via PVTi module of Eclipse 2010 to match the experimental data of single flash vaporization test, differential liberation (DL) experiment and constant composition expansion (CCE) experiment. 9 pseudo-components of crude oil are grouped and their mole fractions are shown in Table 1. The parameters are well matched to meet the accuracy required for simulation ( Table 2). It illustrates that the critical parameters of fluid can reflect the characters of real reservoir fluid.
Slim tube experiment of the crude oil sample is conducted. The result shows that the CO 2 minimum miscible pressure (MMP) of the crude oil and CO 2 is 19.8 MPa (Figure 1 . Four vertical wells are located around them for each model. In this paper, the geometry of fracture extension is assumed to be wire-mesh networks, which forms a rule rectangular fracture network after fracturing. In order to guarantee the convergence in simulation and achieve the rectangular fracture propagation, the grids with width of 0.1m after local grid refinement (LGR) are set to fractures. Vertical wells are also volume fractured to increase their injectivity, which is simulated simply by changing the reservoir permeability around them. Detailed parameters of these numerical models are shown in Table 3.

Development Scenarios Optimization
Different completion measures, including perforated completion, conventional fracturing completion and segmented multi-cluster fracturing completion, are analyzed firstly. Then the optimized well group with superior completion are used to optimize different development modes, such as water flooding, succession CO 2 flooding and WAG of CO 2 flooding. Finally, the parameters, including the formation pressure, production rate, shut-in gas-oil ratio and total gas injection volume, are optimized. Evaluation index is mainly the recovery factor of ten years and the CO 2 -oil draining efficiency. The CO 2 -oil draining efficiency is calculated by the following equation.
Where, is 2 CO E CO 2 -oil draining efficiency, which is the reciprocal of CO 2 utilization factor, is total oil production amount (t), 2 CO T is CO 2 total injection amount (t).

Well completion measures
In order to study the different horizontal well completions impact on development effect, three scenarios are designed. The first scenario is that the horizontal well is completed by perforating. The second scenario is that the horizontal well is completed by conventional fracturing. The third scenario is that the horizontal well is completed by segmented multi-cluster fracturing. The vertical wells are water injection wells,

Numerical simulation model
Based on the tight oil reservoir, three numerical models of fivespot well groups with SRV are built. Model 1, a perforated horizontal well without fracturing located in the middle of the well group and the      the horizontal wells are production wells, and the production control conditions of the three scenarios are the same.
As it can be seen from the Figure 3, the scenario of the horizontal well with SRV has the highest oil recovery, the following is the scenario of the horizontal well with bi-wing fracture, and the scenario of the horizontal well with perforation completion has the lowest recovery. Therefore, the horizontal well with SRV is selected as one of the best well completion measures to develop tight oil reservoir. In the following scenarios, the wells are all volume fractured as wells in model 3.

Development mode
The depletion development mode is usually used to develop tight oil reservoir, which is mainly due to the difficulty in injecting an oildisplacing agent to such tight reservoirs. The vertical and horizontal wells after volume fracturing can obtain SRV around them. The permeability and flow capacity of the reservoir have been greatly improved, which makes it possible to inject an oil-displacing agent to develop tight oil reservoir. Therefore, the water flooding, succession CO 2 flooding and WAG of CO 2 flooding are designed to study their effect on the development tight oil reservoirs. For succession CO 2 flooding and WAG of CO 2 flooding, the same amounts of CO 2 are controlled to inject in these two scenarios.
In Figure 4, the scenario of WAG of CO 2 flooding has the highest oil recovery, while the scenario of water flooding has the lowest recovery.
The oil recovery of WAG of CO 2 flooding is higher than that of the succession CO 2 flooding. One reason is that the same CO 2 amounts are injected in these two scenarios, and both of CO 2 and water can contribute to oil recover in WAG of CO 2 flooding, but there only CO 2 contribute to oil recover in succession CO 2 flooding. The other reason is that after gas breakthrough, the WAG of CO 2 flooding can control the produced gas-oil ratio better than that of succession CO 2 flooding. Considering obtaining the same recovery, it requires fewer CO 2 amount of the WAG of CO 2 flooding than that of succession CO 2 flooding, which reduces the cost of gas flooding. Furthermore, the WAG of CO 2 flooding is better to maintain formation pressure and to reduce the produced gas-oil ratio, so it is selected as the best development mode ( Figure 5).

Production rate
As the production of horizontal well with SRV is controlled by flowing bottom-hole pressure (FBHP), different production rate can be obtained by adjusting the FBHP. In order to study the impact, 4 scenarios with different FBHP are designed, FBHP of which are 8 MPa, 9.3 MPa, 10 MPa and 12 MPa, respectively. Figure 6 shows that the higher production rate (the lower the bottomhole pressure) is, the higher the corresponding oil recovery could obtain. But when the FBHP of production well is less than the saturation pressure, the increased of oil recovery is not that obvious, and the CO 2oil draining efficiency is greatly reduced. Also, the gas breakthrough time would become earlier. Furthermore, it is difficult to maintain the formation pressure around the MMP in the later stage. Therefore the FBHP should not be lower than the saturation, in the other words, the production rate should not be too large. Through the comprehensive analysis of the above, it is preferable to control bottomhole pressure at 9.3 MPa, which is little above saturation pressure (9.25 MPa). When the displacement front reaches the SRV of the horizontal well, the CO 2 starts to breakthrough, then it should be appropriate to increase the water slug to decrease produced gas-oil ratio and maintain formation pressure.

CO 2 injection amount
Based on these following 10 WAG of CO 2 flooding scenarios, the relationship between oil recovery of ten years and CO 2 injection amount is obtained. Figure 6 shows that CO 2 injection volume is proportional to the oil recovery, the larger the CO 2 injection amount is, the higher the oil recovery could obtain.
Ensuring adequate CO 2 gas injection amount is the key to improve oil recovery. Figure 6 shows that oil recovery increases with the increasing of CO 2 injection amount. But when the CO 2 injection amount is larger than 4×104 t, the increase of oil recovery is slow. Therefore, the reasonable total injection amount of 4-4.5×104 t CO 2 is recommended for this five-spot pattern well group.

Conclusion
The permeability and flow capacity of the reservoir have been greatly improved after larger scale SRV measures, which makes it possible to inject an oil-displacing agent to develop of tight oil reservoir. The WAG of CO 2 flooding has better development effect than water flooding or succession CO 2 flooding. The WAG of CO 2 flooding can offset the short comings of low oil displacement efficiency of water flooding, and it can also improve the low sweep efficiency of succession CO 2 flooding. In order to slow down the speed of the CO 2 breakthrough and maintain the formation pressure, it should be appropriate to increase the water  slug when the displacement front reaches the SRV of the horizontal wells. The MMP of CO 2 and crude oil is a key factor for the WAG of CO 2 flooding, it is better to maintain the formation pressure 1-2 MPa higher than MMP. CO 2 injection volume is proportional to the oil recovery, but there is an optimal value.