A Methodology to Evaluate the Gas Migration in Cement Slurries

Abstract

Abstract Gas migration represents 25% of the primary cement jobs failures.For this reason, studies have been done in order to evaluate several properties of cement slurry like fluid loss, permeability, static gel strength, and others. However, the study of these variables has not shown separately a method to prevent the gas migration and which of these properties of cement slurry have to be controlled to avoid such problem.This paper describes a methodology to control the gas migration.The results of this study were validated with field experiences. The methodology consists of the next three steps: the first one is the evaluation of the Flow Potential Factor (FPF) which predicts severity of the problem.Then, the static gel strength is measured as a function of time, obtaining the transition time of the cement slurry.Finally, pressure reduction due to static gel strength versus time is simulated with the Fluid Migration Analyzer (FMA), validating gas migration through cement slurry. Three wells were study in Santa Barbara and San JoaquÍn fields located in Eastern Venezuela. In each well, pore pressure of the gas zone, temperature, gas zone depth and the annular space across the gas zone were used to determine the FPF. The results of this study agree with field experiences, based on quantitative measurements like transition time and FPF.This methodology allows selecting the optimum cement slurry design in a way that prevents industrial accidents and assuring the well life. Introduction The main purpose of the annular cement is to provide an effective zonal isolation for the life of the well in order to oil and gas can be produced safely and economical.One of main problems for achieving this objective is fluid migration in the annular space after well cementing.The main factor preventing the fluid from entering the cement is hydrostatic pressure of cement column and the mud above it.This pressure must be greater than pore pressure of gas-bearing formation to prevent fluid invasion into cement column.Besides, it must not exceed fracturing pressure of the formation to avoid losses.[1] The ability of the cement slurry to transmit hydrostatic pressure, that affects the total hydrostatic pressure of the annular column, is a function of the cement slurry gel strength.The higher the gel strength, the lower is the transmissibility of the annular hydrostatic pressure.When hydrostatic pressure is the same of the pore pressure and the Static Gel Strength (SGS) is not high enough, fluid migration can occur.[1] In recent years much attention has been given to fluid migration in well cementing.In spite of that, there are not any industry recognized standard methods (API or ISO) for evaluate fluid migration problem.For this reason, several methods and definitions have been misunderstood. In this paper a methodology has been developed to prevent fluid migration in well cementing, it consolidates three steps. It takes the FPF that represents the ratio between the absolute maximum pressure restriction possible at a given SGS value and the over pressure (hydrostatic pressure minus the formation pressure).The magnitude of Flow Potential Factor for a specific well condition indicates the difficulty level that the control of fluid flow control will present.Magnitudes vary from 0 to infinity, with increasing value indicating greater potential for flow.[2]