Modelling the Performance of an Iranian Naturally Fractured Reservoir
Authors: Novinpour F. , Sobbi F.A. , Badakhshan A. 
Published: March 2010

Introduction

An Iranian Asmari reservoir with an initial oil-in-place of about 381 million m3 (2.4 billion bbls) and a history of 18 years of oil production is studied in this: work. The field is a typical Iranian fractured carbonate oil reservoir which has a matrix rock of poor quality and a highly permeable fracture network.

The subject of the study was to simulate the reservoir flow performance with an elaborate two-dimensional vertical model capable of simulating non-conventional producing mechanisms such as gravity drainage and block to block interaction. After successful proving of the model capability by reservoir production history, the model was used to predict the reservoir behaviour under natural depletion, and to evaluate two cases of pressure maintenance by gas injection.

Results showed that in both cases of pressure maintenance, an ultimate additional oil production of about 150 million bbls will be achieved over natural depletion. This additional oil production corresponds to approximately 6% of original oil-in-place.

Introduction

The oil reservoir under study is an elongated NW-SE anticline symmetrical along its minor axis located in southwest of Iran. The structure plunge to the southeast is more gentle than to the northwest. The productive area is 19.5 kilometres long and 7.75 kilometres wide at the original WOC located at 1,890 metres subsea. Heavy mud losses during drilling, production tests, and good pressure communication between the wells indicate the existence of an active fracture system. The original reservoir pressure at a depth of 1,600 mss, was measured to be 210.7 bars, about 57 bars above saturation pressure.

Commercial oil production commenced in 1973 at a rate of 3,100 m3/day. The rate was then successively increased as more wells were drilled in the field. During the early time of the field production, a relatively fast pressure drop as shown in Figure 1 was encountered which is characteristic of an undersaturated reservoir. In 1978, the pressure at the crest had dropped below the bubble point pressure causing the liberation of solution gas and the formation of a secondary gas cap.

The field was shut-in from 1979 to 1984 during which 0.2 billion m3 of gas was injected intermittently. During this period the average reservoir pressure increased by approximately 20 bars. Then production resumed at a rate of 6,000-6,500 m3/D with a gas injection rate of 1.4 million m3/D continuously. From 1986 the amount of injected gas was insufficient to maintain the reservoir pressure, resulting in a gradual decline of the reservoir pressure. Figure 2 depicts the reservoir production history.

The producing mechanisms in the fractured reservoirs of Iranian type are substantially different from those in conventional homogeneous reservoirs. Therefore, to study their performance a special mathematical model should be used.

In this study it will be shown that the two-dimensional fractured reservoir model, a "stacked block model," which describes the reservoir as a series of stacked interacting blocks surrounded by fracture space, can be successfully applied to simulate the performance of the reservoir.