Design of Near Well Bore Foam Treatments for High GOR Producers
Authors: Bouts M.N. , de Vries A.S. , Dalland M. , Hanssen J.E. 
Published: December 2007

Foam is being considered for use as a gas-blocking agent in high gas/oil ratio (GOR) oil production wells resulting from gas coning or cusping. Gravity segregation tends to make the placement of conventional water- based foaming agent solutions in the path of the gas problematic. However, non-aqueous foamers, dissolved in a low-density oil-miscible solvent such as kerosene, can be made to rise to the top of an oil zone when injected at gravity-dominated conditions.

A vertical well in the Rabi field (Shell Gabon), suffering from gas coning, was defined as the first field target. Numerical simulations of treatments with a non-aqueous foam revealed two unexpected problems, both caused by the miscibility of foamer solution with crude oil. First, mixing was faster than expected, reducing the achievable depth of penetration. Second, oil flowing directly beneath the foam barrier washed out foamer, which reduced the time of gas blockage. Experiments in a physical sector model verified the occurrence of both phenomena, but also showed that wash-out does not prevent the foam from remaining an effective barrier against coning at constant drawdown. As a second option a treatment in an horizontal well in the same field suffering from gas cusping was also considered.

Introduction

The Rabi field was discovered in 1985 with an estimated STOIIP of some 1.5 109 stb. The field consists of a number of so-called subreservoirs of which the Sand A has a thickness of about 46 m. The oil rim has an overlaying gas cap of about 0.53 1012 scf. The horizontal permeability varies between 0.1-9 Darcy and the reservoir temperature is about 43 C. Further details are given in table 1.

Initially it was planned to develop the oil rim with vertical wells. However, production decline was experienced as a result of beaning-back wells to prevent excessive gas production. Given the limited recovery of vertical wells (average of 4.5 106 bbl), it was estimated that about 180 additional wells would be required to recover remaining reserves. Further developments revealed that drilling horizontal wells would lead to a substantial increase in ultimate recovery per well (2-3 times of that of a vertical well). Based on this it was estimated that the field could be developed by means of 71 horizontal wells. The 1300 m horizontal sections penetrate up to 7 sub-reservoirs that are sometimes separated by area-wide parallel dipping shales. Some sub-reservoirs are highly permeable, resulting in gas cusps into the well. At the current gas/oil ratio (GOR) constraint of 400 scf/bbl (i.e. 100. scf/bbl above SGOR) also these horizontal wells need to be beaned back, see figure 1.

Production at high GOR is a problem in many reservoirs that contain both oil and gas. Apart from reduced oil rates as described above, the negative consequences include: environmental impact resulting from large amounts of flared gas, loss of drive energy and of recoverable oil, downstream processing problems and, ultimately, shutdown if production becomes uneconomic.

Foam can be an attractive GOR-control agent. A foam confined inside the pore network of the rock consists of thin liquid films spanning the pores that make the gas phase discontinuous. This drastically reduces gas mobility essentially without influencing liquid relative permeabilities. Foam is generated by gas displacing a suitable foaming-agent solution. P. 553