Geochemistry: A Powerful tool for Reservoir Monitoring
Authors: Nicolle G. , Boibien C. , Ten-Haven H.L. , Tegelaar E. , Chavagnac Ph. 
Published: December 2007

In an offshore Abu Dhabi field, two main reservoirs are produced commingled. Monitoring the production contribution from each reservoir was previously achieved through either Production Logging or Selective Production Tests. This paper presents the successful introduction of Geochemistry as an alternative tool. This well known technique is based on the analysis of high resolution gas chromatograms of wellhead oil samples. With the help of a dedicated software pairs of peaks are selected, for which the height ratio is significantly different between oil samples of different reservoirs. The split in a mixed oil sample is then derived from the value of these peak ratios. Oil from each reservoir of the field was analyzed, and the two main reservoirs produced commingled were found to have a clearly different signature. A set of samples from commingled wells was then analyzed. The calculated oil splits were found in excellent agreement with values obtained from recent production logging data. The method was successfully extended to the case of wells producing three commingled reservoirs. It is now intended to routinely use this technique, with the following advantages:–Cost effectiveness,–No well intervention or loss of production,–The only applicable method on wells where downhole access is impossible due to mechanical problems,–Valid information even if communication behind casing exists between the two reservoirs.

Introduction

Petroleum geochemistry has been mainly directed towards exploration applications, such as source rock identification, oil-oil and oil-source rock correlations, maturity determinations, etc. The use of geochemistry in development geology and production engineering has however been gaining momentum over the last decade, with the following proven applications: reservoir continuity/heterogeneity determination, reservoir fluid identification (gas/oil/water), and production allocation (e.g. Kaufman et al., 1987, 1990; Baskin and Jones, 1993; Hwang et al. 1994; Baskin et al., 1995; ten Haven and Preston, 1995).

The successful application of geochemistry for production allocation which is presented hereafter was initiated at the end of 1995. The field, located offshore Abu Dhabi comprises 7 reservoir units containing distinct reservoir fluids, as determined by previous PVT studies. Each of these units may in turn be divided into several sublayers, separated by sealing interzones. A schematic of these reservoir units is shown in table 1.

In particular, the Upper Arab reservoirs (Arab A, B+C and D1) are produced through gas-lifted single completions with two or three zones, allowing selective production of each reservoir unit. However, for maximum productivity, most of these wells are producing commingled with all zones open. At end 1996, 22 wells are producing from the Upper Arab, 14 of which producing commingled from B+C and D1, and two others from Arab A, B+C and D1. In addition, three wells are producing commingled the THAMAMA reservoirs.

Regular monitoring of the production allocation per reservoir was achieved by means of Production Logging (PLT) or Selective Production Tests (SPT). A SPT consists in closing successively the lowermost zone(s), while performing a bottom hole flowing pressure measurement at each step, as well as a pressure build-up in the closed zones. The fluid split (oil and water) for each reservoir is then reconstructed, assuming a linear IPR relationship for each reservoir.

The typical PLT or SPT frequency achieved in the past was one every 2 to 3 years. Monthly production is then allocated by assuming that the total liquid split between the reservoirs remains constant, and that the water cut increases equally for all reservoirs.

P. 395^