Behavior and Shape of Gas Kicks in Wellbores

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 128276, ’Behavior and Shape of Gas Kicks in Well Bores,’ by H.F. Spoerker, SPE, OMV E&P, and T. Tuschl, SPE, Leoben University, originally prepared for the 2010 IADC/SPE Drilling Conference and Exhibition, New Orleans, 2-4 February. The paper has not been peer reviewed. The full-length paper describes the driving mechanisms of gas entering into the wellbore and the methodology used for simulating and visualizing the percolation of gas after the well has been shut in. It is part of a multi-year research project with the goal of understanding the wellbore/gas-influx system in more detail. Introduction While of lesser importance for the volumetric and hydraulic calculations required in well-control operations, understanding of shape and distribution of gas bubbles in the annulus during a kick bridges the gap between the hydraulic and the chemical processes happening in the wellbore. Until now, representation of the total gas influx volume as one single-phase, dry-gas “bubble” slowly moving up the shut-in wellbore (i.e., annulus) can be found in many well-control models. Starting in the late 1960s, and especially in the late 1980s and early 1990s, efforts were made to better simulate and measure the behavior of gas kicks in wellbores. These projects could be divided into two groups: (1) the theoretical approach using mathematical concepts and material-balance equations to simulate gas behavior and (2) the practical approach of building several test facilities and flow loops to measure gas behavior under “real” conditions All of these projects attest to the assumption that gas entering the well-bore will not accumulate as one major void fraction. Depending on different factors such as geology, mud rheology, and differential pressure between formation and wellbore, it can be assumed that gas—in contrast to prior assumptions—once having entered the wellbore remains dispersed in the form of multiple small bubbles slowly percolating upward. Formation/Wellbore Interaction Common reservoir-engineering theory explains the movement of gas through a porous medium, in its simplest form adequately represented by the Darcy equation and relating the volumetric flow through a porous medium to the differential pressure applied, the cross-sectional area and length of the rock volume, the dynamic viscosity of the fluid, and the permeability of the rock. Applying Darcy’s law for radial flow, the flow rate for a reservoir/wellbore system at a given differential pressure can be calculated, assuming conventional sandstone reservoirs. While permeabilities for such reservoir rocks are typically in the range of 0.01 to 1 darcy, secondary-porosity fracture systems in limestones and dolomites will behave in a radically different way and can exhibit effective permeabilities of the fracture systems several orders of magnitude higher than those of conventional sandstones.