Underbalanced drilling (UBD) holds several important advantages over conventional drilling technology. These include minimization of formation damage, faster penetration rate, and ability for evaluation of reservoir productivity during the drilling process. As UBD technology matures, it has also been used more and more in different applications. However, many aspects of UBD technology remain poorly understood. The model presented in this paper seeks to understand the mechanisms involved in the transport of cuttings in UBD.

The model simulates the transport of drill cuttings in an annulus of arbitrary eccentricity and includes a wide range of transport phenomena, including cuttings deposition and resuspension, formation, and movement of cuttings bed. The model consists of conservation equations for the fluid and cuttings components in the suspension and the cuttings deposit bed. Interaction between the suspension and the cuttings deposit bed, and between the fluid and cuttings components in the suspension, are incorporated. Solution of the model determines the distribution of fluid and cuttings concentration, velocity, fluid pressure, and velocity profile of cuttings deposit bed at different times.

The model is used to determine the critical transport velocity for different hydrodynamic conditions. Results from the model agree quite closely, qualitatively, with experimental data obtained from a cuttings transport flow loop at the Technology Research Center of the Japan Natl. Oil Corp. (TRC/JNOC)'s Kashiwazaki Test Field in Japan. These results show the importance of slippage in the formation of the cuttings deposit bed. The model is useful in evaluating the minimum flow rate for effective cuttings removal in UBD.


Rommetveit et al.1 developed a numerical model for UBD with coiled tubing. The main features of this transient, 1D model included reservoir-wellbore interaction, alternative geometries for gas injection, and rheology of different fluids, among others. The model consisted of seven mass conservation equations (for free produced gas, free injected gas, mud, dissolved gas, formation oil, formation water, and drill cuttings) and one overall momentum conservation equation. Simulation results showed that a lower gas injection rate was required in the case of gasified drilling fluid, compared with annular gas injection strategy. Gas-oil ratio of the reservoir production into the wellbore was found to have important effects on the ability to maintain downhole conditions underbalanced.

Liu and Medley2 compared results generated from their computer model with results from Chevron's Foamup program and test data. Two different equations of state for foam were derived: downward flow in the drillstring, and upward flow in the annulus. For the upward flow, the model allowed for three phases: gas and liquid in the foam, and solid cuttings. Correspondingly, two mechanical energy equations were obtained. Allowance was also made for the changes in the foam, caused by the influxing of reservoir fluids. The average error between simulation results and Chevron's results and test data was 11.2%. Foam quality was restricted to 0.97 in this model.

Wang et al.3 pointed out the importance of monitoring and controlling downhole pressure, as this affected reservoir fluids influx and, therefore, foam rheological properties. Field data from a Brazilian underbalanced well revealed that the equivalent circulating densities, which were needed in the numerical model, did not reach steady state and fluctuated by up to 50% during the time required to drill one section of pipe. It was postulated that changes in gas and liquid density, void fraction, surface control of choke for two-phase flow, and disturbances attributed to drillstring connections and tripping operations caused the pressure fluctuations.

Wang et al.,4 in a follow-up study, reported the application of their model to two field cases. Dynamic effects such as circulation start-ups and shutdowns, tripping, and gas injection were included. In the first case, nitrogen gas was injected into the annulus as a means of controlling underbalanced conditions. The simulator overpredicted the bottomhole pressure by 3.7%, compared to field measurement. In the second case, nitrogen foam was used as a drilling fluid in a naturally fractured reservoir exhibiting serious loss of circulation. It was found that the simulator overpredicted bottomhole pressures, and the overprediction increased with higher gas injection rates.

Langlinais et al.5 presented experimental data of annular frictional pressure drops for mud-gas mixtures flowing in vertical wells. The mixtures were composed of water-base drilling mud and nitrogen gas. Both the Bingham plastic and power-law models (for slot flow) showed deviation from measured data. Several different two-phase flow correlations were used to calculate the frictional pressure drop. The Hagedorn and Brown correlation, coupled with a power-law rheological model and equivalent diameter, was found to provide the best fit for the experimental data.

Luo et al.6 combined dimensionless groups with experimental data to develop a model for critical flow rates. The model did not consider the effects of penetration rate and drillpipe rotation. The difference between the predicted critical flow rate and laboratory data (for concentric annulus) showed a difference of 15.9%, on the average. Comparison was also performed against field data obtained for three different hole sizes from wells in several North Sea fields. The model was found to overpredict field data for the larger holes, and underpredict in the case of smaller holes. Hole cleaning charts were then developed from numerical results of the model, for different hole angles, maximum rates of penetration, and transport indexes.

Sharma and Chowdhry7 presented an isothermal, steady-state model for annular flow of gas-solids suspension. Some of the main assumptions of the model included: no rotation of inner and outer pipes, and negligible momentum transfer owing to particle-particle collisions and particle-wall collision. A friction coefficient for the solids-gas mixture was defined, based on frictional pressure drop between pipe wall and air, and drag forces between the fluid and solid particles. Results from this model were compared against independently obtained data of simulated air drilling experiments. The difference between calculated friction coefficient and measured value was within 6%; however, experimental data were obtained for a narrow range of particle sizes, and there were no reported pressure data.

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