In the Al Shaheen (ALS) field offshore Qatar, injection wells have generally been back-flowed to remove solids introduced during drilling and stimulation, with the objective of enhancing injectivity. This is considered by many as "best-practice" in the industry. Data from the early back-flow period for injectors and the early production period for producers has been reviewed to try to identify "clean-up" events. Some apparently spontaneous rate increases were observed. Analysis of this data was found to be complicated because during the initial 20 days of production frequent choke size changes were made. Bottom-hole pressure data around the time the wells were opened was also found to be absent in most cases. An attempt to compare the injectivity of a few wells which were not back-flowed with analogues that were was also frustrated, due to variability of permeability and oil viscosity between wells. It was not possible to draw definitive conclusions. A new phase of drilling at ALS provides an opportunity to investigate the efficacy of back-flowing in a pre-mediated fashion. Special provisions will be made during the initial production period to evaluate whether or not the wells clean up. In addition a comparison will be made between the injection performance of pairs of injection wells which are located in areas with similar transport properties. For the injection well pairs, one injector will be back-flowed for a month while the other will be put on injection immediately after stimulation. In this manner it is hoped to demonstrate conclusively whether back-flow of ALS injectors enhances injectivity. Introduction Historically, most Al Shaheen (ALS) water injection wells have been back-flowed before conversion to water injection. Back-flow was done to "clean-up" the injectors and also to produce oil revenue. This paper reviews the possible benefits of back-flow, based on experience with existing wells, and describes steps that will be taken in future to determine the effect of back-flow in a few wells in a carefully controlled environment. The wells in this future trial of back-flowing will each have a "control" well that will not be back-flowed before water injection starts. Although many ALS wells showed apparent "clean-up" events in their early production, it is by no means certain that there was a lasting improvement. The results of back-flow were hard to analyse because, during the early days of back-flow when "clean-up" events were expected to occur, the choke settings were often being changed. In addition, many water injection wells either did not have down-hole pressure gauges, or do not have detailed pressure records to permit more complete analysis of the back-flow period. The back-flow of future water injection wells will generate less revenue than previous back-flow because the new injectors will be in deeper layers and are expected to produce some water almost immediately. Acid stimulation was conducted for the past wells, and this process is considered sound and will be continued. Back-flow is estimated to add some $700,000 to the cost of a water injector because a full production hook-up, including lift-gas, is required. This cost will, however, be more than justified if a 5% increase in injectivity can be shown to result from back-flow.

An opportunity to establish a certain level of confidence in the well models by analysing entire production history or even performing analysis in real time becomes a reality of today. This paper is intended to describe an engineering approach to the analysis that was tailored to the Al Shaheen field to better understand the well performance, gain confidence in the models and identify various well issues and opportunities. The challenge of understanding how wells perform is always associated with comprehensive data mining and significant time spent on analysis and calculations. However the data available is always limited and often requires quite a few assumptions to be made by an engineer when building a representative and reliable well model. All industry standard software packages utilise same or similar well-known concepts and types of analysis from simple equations to more comprehensive algorithms. These are like pieces of the puzzle that can be assembled together to help petroleum engineer to get an idea of how the well should perform in particular circumstances. The way petroleum engineers applying these concepts on daily basis may vary depending on the nature of the problem they are facing and the amount of data they have available. Quite often the fact that the model does not match the reality is used to invalidate existing data and an opportunity to understand that something is happening in the well that is not captured by the model is overlooked. Reasons for this may include an existence of a particular purpose of the well model, level of engineer's experience, skills or imagination, lack of required data or at the end an inability to process the entire production data in an efficient way. The last becomes a real challenge on the fields with large amount of wells and extensive production history. Synergy between adopted analysis and the technology has allowed engineers to gain a much better understanding of the well performance, identify various issues and opportunities and enabled them to keep focus on making decisions as to which wells to optimise and those to troubleshoot to maximise potential of the existing well stock.

The use of water tracers to better understand and improve water flood performance has long been part of the reservoir management toolbox. Its use is even more relevant when applied to complex and heterogeneous reservoirs. This paper describes the planning and implementation phases of a chemical tracer injection pilot in the Al Shaheen oil field, offshore Qatar.In Al Shaheen injection water has historically been used as natural tracer to identify inter-well communication in the field by analysis of the salt and ionic content of produced water. However, this approach has become less effective over time. Identification of short-circuits is getting more complex with the increasing number of wells completed.A tracer pilot including chemical water tracers was designed to identify inter-well communication and flow paths of injected water in a complex area of the field. The geology of this area is highly heterogeneous, with a stacked sequence of carbonate and clastic reservoirs. Numerous features were identified as the possible cause of these inter-well communication paths, including faults and fractures, thief zones, permeable shale and/or absence of shale.The pilot includes two water injection wells completed in two different reservoirs and twenty six oil production wells. This paper reviews the selection of the candidate wells, the operational constraints as well as the detailed analysis and interpretation of the results. Based on this pilot, interwell connection has been proven in most of the area of interest. Tracers have been detected in six of the twenty six wells, with breakthrough times ranging from three to seventy days. Communication between the carbonate and clastic reservoirs was confirmed in two wells. For some of the early samples, anomalous results were obtained. This has been attributed to contamination during sampling at the platform, and was remediated by improving the quality control during sampling.This water tracer pilot paves the way for further application across the field. Ultimately the tracer results will be used in combination with other surveillance techniques to identify by-passed oil and design fitfor-purpose solutions to target higher recovery.

No abstract