Mathematical Space Model Of The Zotti Field In Algeria
Authors: Fagin Richard G. , Jones William W. 
Published: December 2007

The study of the Zotti field was performed as a part of the "The Algerian Hydrocarbon Development part of the "The Algerian Hydrocarbon Development Master Plan" in conjunction with the personnel of SONATRACH to investigate alternative methods of increasing the oil recovery from the field. This study is an example of the detailed work that is being performed in this overall planning effort. Also it performed in this overall planning effort. Also it illustrates how the geological description of a relatively small field with wide spacing, strong water drive, and a small amount of production and pressure drop was significantly improved by the use of a mathematical space model.

Introduction

The Zotti field, sometimes referred to as El Agreb Northeast, is one of three fields formed along a series of faulted anticlines located on the east flank of the Oued Mya basin. The fields are located in the Algerian Sahara approximately 100 kilometers southwest of Hassi Messaoud and 350 kilometers west of the Tunisian border as shown on Figure 1. The Zotti field lies between two fields, El Gassi to the northeast and El Agreb to the southwest.

An early interpretation of the geology of the area was described by Ali and several reinterpretations have been made as new data was added. The reservoir rock is low porosity and permeability sandstones and shales of Cambrian and Ordovician age. No free gas was initially present in the reservoir, and the produced oil has a stock tank gravity of 45 degrees API.

The principal objectives of this study were to determine the amount of oil-in-place and potential recoveries using various methods of development. This was accomplished through detailed geological and engineering studies of the basic data and using a mathematical space model to simulate the field history by adjusting the data within known or expected limits. The first set of history matching runs were designed to determine the sensitivity of the various data. These runs indicated that the relative permeability curves had to be adjusted beyond expected permeability curves had to be adjusted beyond expected limits and the reservoir was substantially larger than what was originally described with well control and geophysical data. The history was matched with the greatly adjusted data and a prediction case was run. The results of this case indicated that the oil recovery percentage would be greater than normally expected. Later a delineation well was drilled which showed that the reservoir was larger than initially described. The new geological data increased the oil-in-place 38 percent and a re-match was made. An excellent agreement exists between the new oil recoveries and published correlations. Both the original and the final matches were good, which indicated that it is important to explore the sensitivity of the data for this, type of reservoir. This was discussed by Hirasaki in his report on estimation of data parameters in the displacement process.

The following paper summarizes the study and reports the unique ability of a space model to study this type of problem and reports the general conclusions of the work.

GEOLOGY

The structural highs of the El Gassi, Zotti and El Agreb fields lie in an area approximately 50 kilometers long and 15 kilometers wide, trending in a northeast-southwest direction. It appears that the reservoirs are connected by a common aquifer. However, pressures calculated by the Reservoir Performance Simulator indicate that there is no Performance Simulator indicate that there is no significant pressure communication between the reservoirs. This is due to the apparent low permeability that exists away from the crests of the structures and the presence of faults between the reservoirs. For all practical purposes the development of these reservoirs can be carried out independently.

The crest of the structure has been truncated by Hercynian erosion and the rock qualities are generally better closer to the erosion surface due to the effects of groundwater solution.