Optimizing the branch number and branch length of radial drilling in high water cut low permeability reservoirs

Optimizing the branch number and branch length of radial drilling in high water cut low permeability reservoirs Xiu-Kun Wang 0 Chuan-Zhi Cui 0 0 China University of Petroleum (East China) , Qingdao , China Radial drilling, also called ultra-short horizontal well, is a new kind stimulation technology, which can be used both in new and old wells to improve the productivity and oil recovery effectively. For the low permeability reservoirs, it had been widely used in practice to effectively solve the unbalanced displacement problems for the waterflooded reservoirs. Applying the multi-layer equilibrium displacement principle, the model of optimizing the radial drilling branch number and length for the high water cut low permeability reservoirs is established, and the corresponding programs are also compiled. Using the reservoir numerical simulation technology, this model is proved to be valid and accurate. This optimization method has been applied in Bonan fifth reservoir of Shengli oilfield, which exhibits quite positive results: higher average production rate and lower average water cut of the radial drilled wells. permeability; Radial drilling; High water cut; Low; Equilibrium displacement - & Xiu-Kun Wang Radial drilling is already not a new technique in the petroleum engineering area. According to the state of art (Marbun et al. 2011), radial drilling is implemented through a special high pressure tube to form the water jet to penetrate and drill several lateral boreholes in one or several layers, which is an effective way to increase the drainage area and improve the oil recovery. Dickinson et al. (1989) first introduced the radial drilling system and predicted the promising expectation of this drilling system. Dickinson et al. (1992) presented two methods of combining water jet drilling and coiled tubing, and discussed their advantages and weaknesses in the practice. Yang et al. (2006) introduced the technique status in China and its application situation in Liaohe oilfield. Bruni et al. (2007) exhibited the Radial drilling technique in Argentina. Ursegov et al. (2008) presented the first results of cyclic steam stimulations of vertical wells with radial horizontal bores in heavy oil carbonates, and the results validated the wide usability of radial drilling technique. And AbdelGhany et al. (2011) stated the first radial drilling well conditions in Egypt. As for all of the researches above, most of them are just focused on the application status or the techniques of the radial drilling system, but almost no research was carried out to probe how to optimize the radial drilling branch length and branch number in terms of reservoir engineering respect, which is quite important for the multi-layer water drive reservoirs. In the low permeability reservoirs, several layers are produced together, due to the differences in the rock properties, the flow abilities of different layers are quite various, which lead to the conflicts of multi-layer, and unbalanced displacement performance. In the practice, engineers tried to solve this problem using the radial drilling technique in the poor property layers to drill several boreholes in order to improve their flow ability, which is proved to be really effective. But, there is still no theoretical basis to guide the engineers to decide how long the lateral wells should be drilled and how many number of the lateral branch bores should be implemented. So, herein according to the equilibrium displacement principle, applying the equivalent flow resistance method, the model of optimizing the radial drilling branch number and length in the high water cut low permeability reservoirs is established. In this paper, we first present the methodology in order to clarify the basic thread of this model. Accordingly, we analyze the productivity of a radial drilling well, and different kinds of flow resistances are presented in this section. In the third section, the equilibrium displacement principle is discussed, and the calculation steps are presented. Last, using the reservoir numerical technology, this method is verified to be valid and accurate. but we do not expect it to be overbalanced, so, the multilayer equilibrium displacement principle is introduced as the optimization basis. The model is established based on the constant pressure difference between the production and injection wells. During the optimization process, the principle of analogy of water and electricity is applied, so the flow resistances are integrant variables that we need to valuate. First, the time spent for the layer without the radial boreholes to get certain water saturation can be obtained, and given the branch number and branch length of the radial drilling wells, the time spent for the layer with radial boreholes to get the same water saturation is also calculated. Then, try to narrow the time difference by adjusting the values of the branch length and branch number, and the ultimate results are the optimal branch length and branch number that are needed to be implemented, aiming at the multi-layer equilibrium displacement. Flow resistance computation Before analyzing the oil–water two phases flow, first, we need to consider the one-phase flow condition. After radial drilling, there are two ways for the formation fluid to flow into the vertical wellbore: (1) formation fluid directly flows into the vertical wellbore; (2) the formation fluid flows through the lateral branch wells then flows into the vertical wellbore (as shown in Fig. 1). Flow resistance of the formation fluid directly flowing into the vertical wellbore This kind of flow pattern is thought to be radial flow. Due to the low permeability of this reservoir, considering the starting pressure gradient, the flow equation is written as Integral Eq. 1 to get According to the analogy of water and electricity, the corresponding flow resistance, Rv, is In order to obtain equilibrium displacement, the poor property layers (lower permeability, thinner thickness, etc.) are implemented with radial drilling. Then flow ability of those layers with the radial boreholes is quite improved, Flow resistance of the formation fluid flowing through the lateral branch wellbores then to the vertical wellbore This kind of flow pattern can be separated into two flow processes (Zhaoxin 2001): the outer flow process and the inner flow process. The outer flow process is thought to be Fig. 1 Two ways that formation fluid flows into the vertical wellbore einh=2 chðn þ igÞ ¼ chn cos g þ ishn sin g: the formation fluid flowing into the multiple fractured well, and the inner process to be a radial flow in the vertical plane with paralleled boundaries. Outer flow process: flowing into the multiple fractured well (shown in Fig. 2). Using the conformal transformation method, choose the transformation function as z n=2 Spreading the imaginary part and the real part of the above equation, we can get By eliminating g, the (...truncated)