Abstract

Mauddud reservoir in the Greater Burgan field is a thin, carbonate reservoir containing light oil in a 10–20 ft target zone with "good" porosity. Matrix permeability is low and natural fracture density can be variable in this reservoir. Thus, this reservoir must be exploited using horizontal wells. In the early 1990's, 16 horizontal wells were drilled in this reservoir. Five more horizontal wells have been drilled in the last 2 years in an effort to scope out the long-term potential of this reservoir.

In conjunction with the drilling of recent horizontal wells, a comprehensive reservoir characterization program culminating into a full-field reservoir simulation model has been completed. The 24 million cell geological model was scaled up to a 9 million cell model at a 50 m x 50 m areal grid level to incorporate flow characteristics of horizontal wells properly in the simulation model. Matrix permeability of the scaled-up model was enhanced using a unique process based on analytical solutions for short fractures and fracture density/orientation mapping for the entire field. This reservoir simulation model has been history-matched for the 13-year production history of 19 horizontal wells using only a global permeability multiplier and water relative permeability curve shape modification. This model has been used in the forecast mode to assess long-term field development opportunity for the Mauddud reservoir. Primary depletion results show that horizontal wells drilled in an intelligent manner in this difficult reservoir hold the key to an economic development of this reservoir.

This paper highlights integrated geological, geophysical, rock mechanics, petrophysical, and reservoir simulation work required to assess the potential of a difficult reservoir characterized by low matrix permeability, thin pay, variable fracture density, and lack of aquifer support. This paper also presents a "stand-alone" full-field, history-matched, parallel simulation model of the Mauddud reservoir for the first time.

Introduction

The Mauddud reservoir is a thin, underdeveloped, low-permeability carbonate reservoir in the Greater Burgan field. References 1 and 2 describe some of the recent field development activities regarding the Mauddud reservoir. This paper highlights integrated geological, geophysical, rock mechanics, petrophysical, and reservoir simulation work required to assess the potential of this difficult reservoir characterized by low matrix permeability, thin pay, variable fracture density, and lack of aquifer support. Study objectives were to:

  1. Develop a full-field, history-matched flow simulation model,

  2. Develop field production potential estimate under primary depletion scenario, and

  3. Obtain a preliminary assessment of the waterflood recovery potential.

In the subsequent sections, geological modeling, formation evaluation, fracture modeling, and reservoir simulation activities related to this study are described in detail.

Geological Modeling

Geological modeling work started by using two structure tops, the Top and Base Mauddud, generated from the interpreted Burgan 3D Seismic Survey. These surfaces were also tied to re-correlated well markers to ensure the thicknesses of the Mauddud section from the wells were reflected in the thicknesses across the reservoir. The average thickness of the Main Mauddud formation across the field is between 30 and 40 ft.

All 16 old Mauddud horizontal wells' and 3 new Mauddud horizontal wells' trajectories were also corrected as part of the geological work. These corrected trajectories required some local adjustments to the Mauddud surfaces to make sure that appropriate lengths of horizontal wells, old and new, fell inside the geological model.

Mauddud geological model uses the same area of interest as was used in prior Greater Burgan models. Individual cells are 50 m x 50 m in the areal direction with 40 cells in the Z-direction. Overall geological model has 599 cells in the X-direction and 1,024 cells in the Y-direction for a total of 24,535,040 cells.

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