Designing Under- and Near-Balanced Coiled-Tubing Drilling by Use of Computer Simulations

Abstract

Summary This paper describes the use of two computer simulation utilities to design underbalanced or near-balanced coiled tubing drilling: one is a steady-state design module and the other is a transient wellbore simulator developed for coiled-tubing operations. The steady-state design module provides various design parameters for a drilling operation, and the transient wellbore simulator predicts the outcome from a particular design for the entire operation. Simulation results show the desired under or near balance can only be achieved by certain combinations of liquid and gas rates when using foam as the drilling fluid or using gas injection through gas-lift mandrel or parasite string. The effects of cuttings loading, depth of injection point and reservoir inflow on the downhole pressure or underbalance are presented and discussed. A procedure for designing under- or near-balanced drilling is described and demonstrated with an example using the computer utilities. A field example is used to show the accuracy of the simulator by comparing the measured and simulated downhole pressures. Introduction It has long been recognized that a key to improving the recovery of reserves is to minimize the reservoir damage created while drilling. Thus, the main objective of drilling close to or under balance is to increase the productivity of the well by reducing formation damage due to the invasion of the formation by the drilling fluid and/or fines. Additional advantages of underbalanced drilling are that higher penetration rates are obtained than with overbalanced drilling, the risk of differential sticking is reduced, and hole cleaning is improved. Coiled tubing is particularly suitable for underbalanced or near-balanced drilling because of the improved well control in a coiled-tubing system and because coiled tubing allows continuous drilling while maintaining the underbalance.1–4 When planning an underbalanced drilling operation, the magnitude of the desired underbalance is a primary consideration. If the downhole pressure is too low, there may be wellbore stability problems. The amount of underbalance also determines the rate of fluid production from the reservoir and the surface facility should be able to handle the production fluid in the return flow. After the desired amount of underbalance is determined, the next step is to determine how to achieve this underbalance. For normal or high pressure reservoirs, underbalance can be achieved by using water, brine or diesel as the drilling fluid. For low pressure reservoirs, nitrogen or produced gas is used to aerate the drilling fluid and reduce downhole pressure. Two injection methods are often used. One is pumping foam or nitrified fluid down the coiled tubing and into the annulus, and the other is injecting nitrogen or produced gas through a parasite string or through existing gas lift mandrels to aerate the liquid column in the annulus. The advantages and disadvantages of using foam and gas injection are discussed in Refs. 5 and 6. Aerated or foam drilling fluids are compressible. Downhole pressure or the amount of underbalance while circulating is more difficult to calculate for compressible fluids, and computer simulation tools therefore become necessary when designing under- or near-balanced drilling using compressible fluids.7–9 This paper is concerned with how to achieve the desired under or near balance when using foam or gas injection. Downhole pressure behaviors for various drilling conditions are studied using two computer simulation utilities. A design procedure is presented and demonstrated with a simulation example. Computer Simulation Overview When designing an under or near balanced drilling operation, questions like those listed below often need to be answered:What are the gas and liquid pump rates for achieving the desired under or near balance?What are the gas and liquid pump rates for achieving efficient hole cleaning?How much reservoir fluid will be produced in an underbalanced condition?How does the produced fluid and cuttings loading affect the downhole pressure?What is the maximum rate we can pump without exceeding the pressure limit of the coiled tubing or other well equipment? Two PC based computer utilities have been developed to assist the design of coiled tubing operations. One is a steady-state design aids module (DAM) and the other is a transient wellbore simulator.10,11 They are well suited for answering these questions when designing under and near-balanced coiled-tubing drilling operations. The DAM calculates the coiled-tubing circulation pressure, downhole pressure, foam quality, flow rate and solids concentration for given wellbore and coiled-tubing conditions. It provides liquid and gas rates for achieving the desired downhole pressure based on simplified steady-state conditions. The results are presented in the form of sensitivity plots and show the trend of change of the parameters, which is particularly useful when there is uncertainty in the design conditions. A pump schedule, as well as coiled-tubing size, fluid types, and injection method, can be determined based on the simulation results from the DAM. The operation design can then be tested using the wellbore simulator. The wellbore simulator predicts the transient results of pressure, velocity, cuttings removal, and foam quality in an operation when a designed pump schedule is executed. It includes the effect of the coiled-tubing movement, cuttings transport, inflow or fluid loss due to the reservoir, and change in pumping conditions during the operation. Based on the transient results, all requirements and limits can be verified for the entire operation.

An Integrated Sand Cleanout System by Employing Jet Pumps

“…Furthermore, a larger sand ultimate settling velocity demands a larger velocity and rate of the carrier fluid, according to Equations (7) and (8). During the sand cleanout operation, water was used as the working fluid.…”