Abstract

Gas migration through cement columns has been a problem of the oil and gas industry for many years. The most problematic areas in Iranian South Pars Field operations for gas migration are in gas wells; which causes failure of about 30 % of the primary cement jobs. Gas migration in these gas wells occurs when drilling through the Kangan and Dalan formations. To control gas influx, cement densities required to successfully cement this zone could be as high as 120 pcf. Specially designed cements were used to "zone isolate" the high pressure coming from the formation. High density/low fluid loss cements were used in the past with limited success. The objective of this study was to develop an effective cement formulation to control deep gas migration during primary cementing. To measure major slurry properties lab equipments were utilized for this study. A static gel strength analyzer (SGSA) is used to determine the static gel strength of formulated well cements to confirm the data from previous experiments on the slurry (rheology, filtration, and free water). Results from the SGSA were utilized to reduce the gas migration problem. This study also discusses the individual components necessary for gas-leak elimination. In addition, clearly addresses the gas migration problems related to cementing operations.

Introduction

Oil and gas well cementing technology is a mixture of many interdependent scientific and engineering disciplines, including chemistry, geology, physics, and petroleum, mechanical, and electrical engineering. Each of these is essential to achieve the primary goal of well cementing-zonal isolation. The major objective of primary cementing is always to provide zonal isolation and exclude fluids such as water or gas in one zone from oil in another zone. Without complete zonal isolation in the wellbore, the well may never reach its full producing potential.

Remedial work required to repair a faulty cementing job may do irreparable harm to the producing formation. In addition to the possibility of lost reserves and lower producing rates, production start-up is delayed. Other problems may arise, suchas not being able to confine stimulation treatments to the producing zone, or confining secondary and tertiary fields to the pay zone.

With regard to industry needs, cement slurry must be designed properly to fulfill the requirements of each well condition. To design a cement slurry formulation, several factors should be considered, including well depth, temperature, mud-column pressure, viscosity and water content of cement slurries, pumping, or thickening, time, compressive strength, quality of available mixing water, compatibility with drilling fluid and spacers, density, lost Circulation and filtration control.

Theory

Perhaps the most important concern for deeper high-pressure oil and gas wells has been the control of flow after cementing. Without proper slurry design, gas can invade and flow through the cement matrix during the WOC time. This gas must be prevented from invading the cement. Failure to prevent gas migration can cause such problems as high annular pressures at the surface, high water/gas cuts, blowouts, poor zonal isolation, loss of gas to non-productive zones, poor stimulation, low producing rates, etc. All of these are costly to correct. It is generally acknowledged in the industry that the mechanism that allows gas invasion into the cement matrix is the gel strength development of the slurry as it changes from a liquid to a solid. In this condition, the cement loses its ability to transmit hydrostatic pressure, and gas invasion may occur. Other mechanisms include excessive fluid loss, bridging, cement body permeability and the formation of microannuli.

There are several successful methods to control gas migration each with its advantages. Usually a combination of methods works best. In selecting optimum methods for controlling gas migration, many well conditions must be considered: formation pressure, permeability, gas flow rate, and bottom-hole temperature; wellbore geometry, well deviation, height of the cement column, and formation fracture pressure.

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