Model Simulates Gas Kicks in Nonaqueous Drilling Fluids
Authors: Carpenter Chris 
Published: December 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 189606, “Gas Kicks in Nonaqueous Drilling Fluids: A Well-Control Challenge,” by Z. Ma, A. Karimi Vajargah, D. Chen, and E. Van Oort, SPE, The University of Texas at Austin, and R. May, J.D. MacPherson, SPE, G. Becker, SPE, and D. Curry, SPE, Baker Hughes, a GE Company, prepared for the 2018 IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, 6–8 March. The paper has not been peer reviewed.
Nonaqueous drilling fluids, such as synthetic-based and oil-based mud (SBM and OBM, respectively), are used frequently to drill one or more sections of a well to reduce drilling problems such as shale sloughing, wellbore stability, and stuck pipe. However, solubility of formation gas in such fluids makes early gas detection, and the well-control process, very challenging. This is of particular concern in deep offshore wells. This paper presents a novel and comprehensive hydraulic model to simulate a gas kick in nonaqueous drilling fluids.
Introduction
Early kick detection becomes cumbersome when a nonaqueous drilling fluid is used. This is because of dissolution of gas in the nonpolar base fluid. Although gas entrance into the well still will result in increased flow-out on surface, the flow signature will be less pronounced in comparison with a similar gas kick in water-based mud (WBM). Moreover, the emergence of gas from the solution may often only begin when the gas/drilling-fluid mixture is relatively close to the surface, which leaves the crew only a short period to react before gas reaches the surface. Constant-bottomhole-pressure/managed-pressure drilling (CBHP-MPD) offers the benefit of delaying the bubblepoint until the kick has passed the choke, thereby not allowing the kick to come out of solution in the actual well.
Background
Gas-Kick-Simulation Multiphase-Flow Modeling. The drift-flux model (DFM) used in this work is a mechanistic model consisting of separate mass-balance equations and a combined momentum-balance equation, together with several closure algebraic equations and a slip law. To obtain the velocity of each phase, the slip law is used in conjunction with the combined momentum-balance equation.
Solubility in oil-based drilling fluids is directly proportional to pressure, gas specific gravity, and base-oil volume in the drilling fluid and is inversely proportional to temperature, drilling-fluid solids content, and brine and emulsifier volumes in the drilling fluid. For a given drilling-fluid volume, the relative volumes of base oil, brine, and emulsifier in the drilling fluid can be determined from a standard retort analysis. However, the most-reliable method to determine this crucial simulation parameter is to conduct pressure/volume/temperature (PVT) testing for each mixture under downhole conditions.
DFM Formulation. Mathematical modeling of a proposed 1D DFM to describe the multiphase-flow dynamics for non-aqueous drilling fluids is discussed in detail in the complete paper. The current formulation takes into account the mass transfer between the liquid (drilling fluid) and formation/injected gas. The model consists of two sets of partial differential equations for two different cases: when gas is completely dissolved in the liquid (no free gas) and when dissolved and free gas coexist in the system.