Water Management in North 'Ain Dar, Saudi Arabia

Abstract

Abstract This paper presents a study on a six-year water management strategy in North 'Ain Dar to reduce water production, prolong well life and enhance oil recovery. This field has been under peripheral injection for more than thirty years. Three strategies were implemented during the past five years to achieve optimal water management:production optimization,rigless water shutoff jobs, andhorizontal drilling. The success of these strategies can be attributed to a full understanding of the drive mechanisms that control fluid transport in the reservoir. This paper presents these mechanisms and their impact on water management strategies. The effectiveness of these strategies have been evaluated and supported by field data. Horizontal drilling and rigless water shut-off proved to be effective techniques to control water production and enhance recovery in this gravity dominated system. Since 1999, production optimization has also played a significant role in controlling the water production and maintaining a constant water cut. Introduction Field History: This field represents one of the most mature parts of Ghawar. It was discovered in 1948 and regular oil production began in 1951. The oil grade is Arabian light with an average API of 34º and a solution GOR (Gas-Oil-Ratio) of 550 SCF/STB. This field has a modest natural water drive support, thus peripheral water injection was initiated to provide full pressure maintenance in 1968. Initially, water injection was conducted by gravity water injection. This was replaced by power water injection to provide flexibility in controlling the waterflood front propagation. Reservoir Geology: This field has more than four oil and gas bearing reservoirs. The focus of this study is the Arab-D reservoir, which is the most prolific oil bearing formation. In this field, Arab-D is characterized as a folded anticline consisting primary of Jurassic carbonate. The diagenetic effects on the Arab-D sediments are minimal and the calcite cementation is rare and barely coats grains surface.[1] Matrix porosity and permeability averaged 25% and 600md, respectively. Historical Performance: Fig. 1 shows the overall production history of the field. The field has maintained the capability of producing at a high rate for almost 50 years except for the mid-eighties due to low demand. The water production started in the late seventies and increased moderately to 42% at the beginning of year 2005. The main focus of this paper is the last six years. Since 1999, the oil and water production, water injection, and reservoir pressure were kept effectively constant as shown in Fig. 2. This sound behavior is a result of active strategies taken during the subject period. Those strategies are in alignment with the rigorous logic of reservoir management tenets practiced by Saudi Aramco. As defined by Saleri[2], the tenets include thoughts, principles, processes and practices by which the field is managed. Reservoir Surveillance & Fluid Mechanics The field has been under a yearly comprehensive surveillance plan. This plan dictates full areal coverage for logs and static bottom-hole pressure (SBHP) surveys as shown in Figs. 3 and 4. Logs, such as Formation Analysis Log (FAL), Production Logging Tool (PLT), Carbon/Oxygen (C/O), and Thermal Decay Time (TDT), are required to ensure vertical sweep conformance and infer sweep mechanism. SBHPs are required to ensure that the reservoir pressure is above the bubble point pressure. In addition, a strategic surveillance materplan (SSM) has been initiated to further ensure accurate determination of remaining oil saturation (ROS) and pressure. The SSM will be discussed later in this paper. It is imperative to mention that the water management strategies were not on the expense of sweep conformance. Areal sweep conformance can be easily demonstrated by the even movement of the flood-front as shown in Fig. 5. While vertical conformance can be supported by FAL results for wells drilled behind the flood front. Fig. 6 shows the FAL results for A1 and 2, where both wells exhibit an excellent vertical conformance.