Case Study: Water Shut-off Mechanism in Small, Remote Platform-Process & Challenge

Abstract

Malong is part of an integrated marginal field developed in 1999 called MASA. Hydrocarbon is evacuated to a nearby Floating Production, Storage and Offloading (FPSO) facility for export. Its produced water system is currently being used up to its maximum design limit and hindering additional oil production. Malong main reservoir J2 contributes about 97% of the total production. Currently, 7 wells are producing at 7.7 kbopd with average water cut of 70 percent. 88 percent of 18 kbwpd comes from 5 deviated wells while the rest is from 2 horizontal wells. J2 is a water injected reservoir with moderate to strong aquifer. This high permeability sand is distributed fairly well across the field and is subdivided into upper and lower lobes. A comprehensive diagnostic of water flow entry mechanisms begins with compilation of critical information such as geological and reservoir data, production and injection data, cased hole data and well history. Subsequently, several analytical techniques such as material balance analysis, water control diagnostic plot, bubble maps and nodal analysis are applied. The water production mechanisms are primarily dynamic contact movement due to production and possibly accompanied by flow behind casing. In all the wells, the lower lobe is mostly flushed leaving some oil from upper lobe only. Screening of suitable water shut-off methods gets complex considering tubing integrity issue, i.e. multiple tubing leaks, poor cement bond and possible deep invasion due to high permeability formation. Detail evaluation is performed to weigh the pros and cons of mechanical type (cement squeeze) and chemical type (polymer gel). Finally, polymer gel is selected on the basis of less operational risk, controlled depth of invasion and easier removal. Additionally, coil tubing with packer is selected as the deployment method to ensure accurate placement of gel and to evade chemical loss through leaks. Introduction This paper is intended to outline the processes and challenges that were encountered in understanding the produced water problem and in designing the appropriate water shut-off methods considering the physical limitations of facilities as well as tubing integrity. The work has actually started since end of 2004 with downhole data gathering for some of the wells. More efforts were put into understanding the problem and coming up with practical solutions as more water are produced and putting higher constraints on the facilities. Understanding the Problems Without doubt, the first step would be to understand the problems in hand. Water has been produced together with oil since the beginning and the facilities on FPSO are designed to handle the produced water without jeopardizing the oil production. This is anticipated since Malong J2 reservoir driving mechanism is moderate to strong aquifer drive with water injection for pressure maintenance. However, the actual water produced has exceeded the design capacity and thus prompted effective water control measures to be in place. The reservoir water flow mechanism needs to be well understood to enable effective implementation of water control measures. In addition, the economics of selected measures are imperative so as not to erode the profitability of the operation.