Reservoir Optimization and Monitoring: Mauddud Reservoir - Bahrain Field
Authors: Al-Muftah Ali Ebrahim , Vargas William Orlando , Murty C.R.K. , Abdulwahab Ayda 
Published: February 2016

For a matured oil field like Bahrain Field with a long production history, it is required to identify underperforming areas, infill wells and upgrade the reserves.

This paper describes the application of a practical process (1) to develop systematic workflow for production optimization and reservoir analysis; (2) Identify and highlight reservoir trends, patterns and anomalies; (3) Identify and highlight the under performing wells/areas and recommend solutions, and (4) Identify essential patterns for consideration in overall development plan. It is required to quickly adopt assessment methods for such a mature field. The area used for the study consists of 431 wells in Mauddud reservoir which is one of the major producing zones. The challenge was to evaluate large data sets in a short time and cost-effective manner.

The technique uses a streamlined workflow of reservoir assessment processes, which require a sequence of data gathering, formatting and validation through combining the data with several processes associated with both the static and the dynamic model of the reservoir. Quick interpretations of these models generate opportunity regions, re-completions and workover candidates, and new infill potential in the reservoir. Based on the processes run in the Mauddud zones it was possible to understand rapidly the reservoir performance and main issues associated with field development (water production, gas injection, potential transfer areas). In addition,

underperforming wells/areas and potentially undrained areas (high remaining reserves zones with low water cut and low Gas Oil Ratio) were identified with certainty in a timely manner. As a result of these techniques, the development drilling program was suitably adopted to realize an efficient reservoir management process for developing the field and helped in decreasing decline rate and increasing the recovery.

Introduction:

The Bahrain field, the first discovery in Arabian Gulf region, has been on production since 1933. It is an asymmetrical anticline trending in the North-South direction. The field is a multi-stack carbonate and sandstone reservoirs with 16 oil and 6 gas reservoirs. Most of them are carbonate reservoirs. The nature of the fluids varies from tarry oil in Aruma zone to dry gas in the Khuff zones (Figure-1). The geology of the field is extremely complex with a large number of faults especially in the Wasia group formations, which contain the major oil reservoir of the Bahrain field, Mauddud (Figure-2).

The Bahrain zones, of the Lower Cretaceous age, are the most important oil-producing group found in this field. The most important zone within this group, and the subject of this paper, is Mauddud. The reservoir rock consists of white to light gray, fine- to medium-grained, clean bioclastic limestone. Bioclastic packstones and wackestones dominate. The degree of lithification varies, ranging from moderately soft to hard. Gross thickness varies slightly from 102 to 116 f, all of which is considered net pay. Average porosity and permeability are 31% and 65 md respectively. The middle 50 ft has the highest porosity and permeability. The basel 30 to 40 ft is slightly different in character and appears to have a lower specific productivity index. Average initial water saturation was 6% in the upper section (Ba) and 11% in the basal section (Bb).

The reservoir oil was highly undersaturated, Pb @ 450 psig as compared to original reservoir pressure of 1236 psig at 1900 ft subsea. The oil has a solution gas/oil ratio (GOR) of 128 scf/bbl and a density of 0.8556 g/cm3. This major oil zone which is an oil wet system, has been on gas injection since 1938. Over the years of injection a secondary gas cap has developed. The dominant recovery mechanism is gravity drainage with crestal injection of gas from the Khuff zones. The reservoir being heterogeneous and complex structure with a large number of faults, most of the injected gas gets produced. Also significant fluid transfers take place to the zones above and below. As this complex situation can be understood well only by simulation, a simulation model was constructed by utilizing an updated geological model after a recent 3 D seismic survey of the field.