DETERMINATION OF WATER SATURATION IN AN OIL RESERVOIR

0
213

ABSTRACT

This study was designed to determine the amount of water saturation in an oil reservoir. For each well, the following logs were collected; resistivity log, sonic log, formation density compensated log, compensated neutron log and gamma ray log. These in situ well logs were subjected to well log analysis and interpretation methods. The following Petrophysical parameters; porosity, permeability, water saturation, reservoir thickness and volume of shale were estimated for each hydrocarbon-bearing zone delineated for each well. The data obtained have been analyzed and interpreted quantitatively, to assess the hydrocarbon potential of each well. The mean estimates for porosity and permeability of all the oil-bearing zones delineated, range from 23.75 % to 34 % for porosity and 65 md to 714.7 md for permeability, compared to mean porosity range of 8.5 – 23.15 % and permeability range of 6.83× 10-2 md to 5.99 md of the gas zones. The results of the well logs interpretation suggest that oil- bearing zones are much more porous and permeable than the gas-bearing zones. Thiswas evident throughout the wells. The estimates for the water saturation of hydrocarbon-bearing zones range from 6 % to 63.6 % indicating good hydrocarbon saturation potential. The reservoir thickness estimated for all the oil-bearing zones delineated range from 2 m to 40 m while the gas-bearing zones range from 4 m to 10 m. The Western Basin low estimated reservoir thicknesses for hydrocarbon-bearing zone suggests that reservoir reserves potentials are not sufficient, but the range 8 % to 36.8 % of volume of shale estimated for the hydrocarbon-bearing zones of the wells suggests that sandstone lithology of the Western Basin is clean. The Western Basin proved to be a high capital venture, as far as drilling is concern due to abnormal high pressure zones detected in almost all the wellsstudied.

CHAPTER 1 INTRODUCTION

1.1 Water saturation in oil reservoirs

Oil reservoirs are the main properties of exploration and production companies. Most of the reservoirs at least consist of two different phases of liquids. These phases are gas-water or oil-water. Some of the reservoirs have all of the three phases of gas, oil and water ( Dandekar, 2009). Sandstone, as the most famous reservoir rock, has many spaces to reserve hydrocarbon. Carbonate rocks are also important kips to reserve considerable quantities of hydrocarbon.

One of the most important tasks in reservoir engineering is determining the characteristics of reservoirs to estimate their performance in future. Without a proper and accurate characterization, Many prominent errors can enter the reservoir performance prediction. These errors can finally lead to loss the noticeable values of hydrocarbons in extracting process.

Reservoir characterization is a process of describing reservoir properties using all the available data to provide reliable reservoir models for accurate reservoir performance prediction (Jong, 2015). This process can be either qualitative or quantitative. In qualitative characterization the quality of rock is evaluated in order to see if it can be a reservoir rock. Quantitative reservoir characterization is the process of numeral statement of some characteristics in reservoir such as permeability, porosity, saturation, pressure and pores size. Suitable data used in this characterization are cores, logs, production and seismic data.

One of the most important parameters in an oil reservoir characterization is the water saturation. Kamel and Mabrouk (2012) assumed that all void spaces in a reservoir consist of water and hydrocarbon, therefore:

So = 1-Sw                                                                                                          (1)

where So is oil saturation and Sw is water saturation.

1.1.2 Determination of water saturation

This section takes a brief overview of an historic method which is used to determine the saturation in an oil reservoir.

In 2008, Archie determined that it is possible to determine the value of water saturation for a clean (non shaly) formation using porosity and electrical resistivity values of the formation (Archie, 2008):

In his equation, SW was the water saturation of the clean formation, Rt is the corrected total electrical resistivity of the rock obtained by porosity logs such as acoustic or density,  Rw  is  the  water  resistivity  obtained  from  self-potential (SP) logs or production tests, is a constant which depends on rock type and tortuosity of the fluid path,  and  m  and  n  are  cementation  and  saturation exponents which are constant. The value of 0.6 was assumed for unconsolidated sand stones, 0.8 for consolidated sand stones and one for carbonates (Kamel and Mabrouk, 2012).

Cementation exponent can get values between 1.6 to  2.4, but generally it is equal to 2 and the value of saturation exponent is different from 1.7 to 2.2 (Moradzadeh and Ghavami,2011).It should be mentioned that there are other well logs which can give the values of water saturation. As an example by combining the density log with hydrogen index obtained from NMR log, it is possible to determine the water saturation value. This procedure is only sensitive to the gas saturations and can be applied in the regions near the wells (Ellis and Singer, 2017; Holstein, 2017). Therefore the most important well logs in water saturation measurements are resistivity logs. In accordance with these findings, this research work is focused on the determination of water saturation using data from logs.

1.2    Statement of the Problem

The success of any oil exploration exercise depends greatly on the accuracy of the determined petro-physical parameters. Water saturation is one of such parameters and it is a major factor in most petroleum related equations. It is in light of the need for accuracy with regard to petroleum activities that this research aims to determine the efficiency of well logging as a medium for determining water saturation in oil reservoirs.

DETERMINATION OF WATER SATURATION IN AN OIL RESERVOIR