(203ac) Isothermal CFD Modeling of Annular Multiphase Flows During Underbalanced Drilling (UBD) in Oil Reservoirs

Numerous oil fields are situated within carbonate formations which present significant challenges for production as well as for drilling purposes. Carbonate reservoirs are multi-facetted at every level and present ubiquitous technical problems: the risk of lost circulation zones is addressed via various techniques, among which Underbalanced Drilling (UBD) is prevalent. As UBD occurs by means of two-phase fluids (e.g. gasified or foam liquids), a major issue of concern is the efficiency, stability and cost of cuttings transport from the rock formation towards the surface in such a multiphase environment. The reliability and safety of the drilling process greatly depend on understanding how rock cuttings enter the fluid flow, how they are stably suspended and how they can be rapidly removed. In many situations, UBD is accomplished with the deployment of coiled tubing: therein, the conventional drill pipe is replaced by a continuous pipe in a coil, which allows for more efficient drilling under special conditions. Critical technical problems thus encompass cuttings slip velocity, rock formation layer modeling for cuttings movement to the annular space and pressure drop estimation at different inclinations, particularly for flow within horizontal wells. The implementation of UBD has been very encouraging in North Africa and the Middle East as a key enabling technology towards adding significant incremental reserves, especially when combined with horizontal wells and enhanced oil recovery processes.

The flow geometry in all UBD situations is an annulus at vertical, inclined or horizontal orientation. While technical challenges for cuttings transport related to vertical configurations have been mostly resolved, there are several unresolved problems for the flow of two- and three-phase solid–liquid (SL) and solid–liquid–gas (SLG) mixtures along horizontal and inclined conduits (Tomren et al., 1986; Li & Walker, 2001); consequently, research efforts continue towards appropriately detailed, realistic and technically successful modeling of cuttings transport along horizontal and inclined annuli, because this operating principle enables inherently efficient and safe in-situ design and operation of horizontal drilling (Cheng & Wang, 2008). Solid-liquid flows in annuli are characterized either directly (visually) or indirectly (using normally pressure variations) as one of several geometrical configurations or flow patterns. Liquid velocity, solids loading, physical properties, inclination angle and conduit shape and size are the main determinant parameters (Govier & Aziz, 1972). A detailed description of the flow patterns for liquid-solid flows in a concentric annulus has been recently published (Kelessidis & Bandelis, 2004).

Computational Fluid Dynamics (CFD) modeling of formation rock cuttings transport during UBD drilling operations has strong descriptive and predictive potential which has not been hitherto fully harnessed to analyze, design, optimize and streamline safe UBD drilling operations, except in very few cases (Nakagawa et al., 1999; Li & Kuru, 2003). Research studies for gas-solid-liquid flows (the fundamental mechanism of UBD cuttings transport) have been published in the past, but most are not concerned with the actual and credible elucidation of flow patterns and cuttings distributions. Despite the considerable theoretical (multiphase closure equations) and computational (algorithmic robustness and convergence) challenges, commercial and/or open-source CFD codes can address such problems at an acceptable level of detail, thus providing spatiotemporal distributions of all key UBD state variables of interest. This paper aims to present and discuss a number of concrete case studies on steady-state multiphase CFD modeling and resulting UBD state variable (pressure, slip velocity, solids loading) distributions as a function of annular configuration geometry and horizontal (UBD pipe) position.

LITERATURE REFERENCES

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