WO2019219153A2 - Estimation of free water level and water-oil contact - Google Patents

Abstract

This invention proposes a method for estimating the level of a Free Water Level (FWL) and Water-Oil Contact (OWC) based on finding two intersecting trends of the pressure gradient (1) for oil (non-wetting phase) and (2) water (wetting phase) in the oil interval based on the results of drilling only one exploration well, penetrating only the oil-saturated part of the reservoir and not penetrating the water-saturated part of the reservoir and OWC. The pressure gradient for oil (non-wetting phase) is based on well tests while drilling a well (MWD) with a formation wireline tester, which allow determining the pore pressure of the reservoir in the flowing or mobile phase (oil) along the borehole, varying with depth. The pressure gradient for connate water (wetting phase) in the oil-saturated zone of the reservoir and its trend along the reservoir height to the Water-Oil Contact (OWC) is determined by measuring the capillary pressure curve of the reservoir varying in height according to petrophysical data and special core laboratory studies sampled from the oil interval in the exploration well. This method allows one to determine the position of the FWL and OWC based on the results of drilling and testing only one exploration well that penetrated only the oil zone of the reservoir and aid not penetrate the OWC. Thus, there is no need to drill additional wells to determine the position of the OWC and assess the oil reserves of the field.

Description

Estimation of Free Water Level and Water-Oil Contact

Process

This invention relates to a method to estimate a Free Water Level (FWL) and Water-Oil Contact (WOC) in the oil and gas fields using data available from the measurement in one exploration well.

Introduction

In the exploration well drilled a wireline formation tester can provide accurate pore pressure measurements for mobile phase in the reservoir pores . In the open hole section of the well the wireline

pressure measurement is acquired by inserting a probe into the borehole wall and performing a mini drawdown and buildup by withdrawing a small amount of formation fluid and then waiting for the pressure to build up to the formation pore pressure. Such Measurements While Drilling (MWD) can provide formation pressures along the borehole, thereby giving a measure of pressure with depth.

The trends in formation pressure with depth in the oil saturated and water saturated zones are different due to pressure differences in the oil { assumed in most of the cases as non-wetting phase in the reservoir) and water ( assumed wetting phase) phases controlled by capillary forces. Crossing of these pressure gradient trends indicates a fluid contact or FWL (Figure 1).

Capillary pressure (P_c) is the pressure difference across the interface between two immiscible, wetting and non-wetting fluids, like water and oil in the reservoir porous medium:

Pc: — P_non-wetting phase - P_{wettlng phase}

For gas-oil system in the formation, oil is typically the wetting phase and gas is non-wetting phase .

Capillary pressure for a specific reservoir can be measured in a series of laboratory experiments on reservoir core samples in one of the following ways:

• Porous diaphragm method

• Mercury injection method

• Centrifuge method

^• Dynamic method The shape of the capillary pressure curve depends on pore sizes, their distribution and fluid properties, Figure 2. The permeability is lower in the smaller pores having higher capillary pressure.

Different rock types in the reservoir have

different distributions of porosity, permeability, pore geometry, grain size. These differences may cause different pressure gradient trends for the wetting phase .

Petrophysical data acquired during Logging While Drilling (LWD) measurements in the exploration well provide information about saturations in the reservoir. Log interpretation analysis may allow to establish categories of rock types with different capillary pressure curves.

Invention

Knowing the measured pore pressure gradient in the interval with mobile non wetting phase, one can

calculate a pressure gradient for the wetting phase present in this interval of the formation at irreducible saturation. This pressure gradient for the wetting phase can be calculated as follows:

P_{wetting phase (h)} =P_{non-wetting phase (h)} — P_c ( h )

The Pc values along the height of the reservoir may be calculated from the water saturation values using measured capillary pressure curve, Pc function (Figure 2) . Required values of irreducible water saturation in the oil zone of the reservoir can be derived from the geophisical data, resitivity log interpretation.

Two established pressure gradient trends for oil (non-wetting phase) and water (wetting phase) in the oil zone may be used to estimate a position of the FWL, Figure 3.

If the reservoir section has different rock types or non communicating stratified layers, the appropriate capillary pressure curve should be applied to each rock type. In this case different non communicating reservoir rock zones may have different FWLs .

Claims

Estimation of Free Water Level and Water-Oil Contact Claim

1. A method to estimate FWL and WOC using data from only one exploration well that penetrated only the oil column of the reservoir and did not penetrate the water-saturated part of the reservoir and the WOC by:

• Interpreting well test (wireline formation

tester) and petrophysical data from core analysis (capillary pressure measurements) and log data

( saturations ) .

* Determining pressure gradient trends (1) for oil (non-wetting phase) based on the results of wireline formation tester (MWD) and (2) for water (wetting phase) in the oil interval from the capillary pressure curve established for the reservoir height based on petrophysical data and special laboratory studies of the core sampled from the oil saturated zone.

Finding intercepting point of pressure

gradient trends (1) for oil (non-wetting phase) and (2) for water (wetting phase that indicates the FWL in the reservoir and position of the WOC.