As drilling and completion technology has advanced dramatically, developing new oil fields faces new opportunities of well structure selection, especially for the fields that are sensitive to issues such as environmental conservation, cost effectiveness, safety control, and well management. Horizontal, multi-branching and multilateral technologies have been used in many field applications all over the world to enhance the reservoir recovery in a cost-effective way. When the well structures become complex, producing from these "fancy wells" could become difficult, and sometimes, detrimental to overall recovery. To study the strategies for optimizing multilateral wells, we have developed a mathematical model to predict well performance for horizontal and multilateral wells using coupled multiphase models of wellbore/reservoir flow. The flow distribution along the laterals is predicted as a function of tubing head pressure (deliverability of the well), and overall well performance as a function of time is predicted by a simple material balance relationship.
A common problem for such wells is that commingled production from multilateral branches can often lead to crossflow from one reservoir compartment to another. The likelihood of crossflow depends on initial reservoir conditions, but also very much on the operating conditions of the well. Deleterious interactions between commingled reservoirs accessed by multilateral wells must be reliably preventable for these types of completions to have broad application. Some case studies and hypothetical examples are presented in the paper to show the procedures for optimizing the well structure and well performance. Production strategies that would help to eliminate crossflow are discussed.
To study the effect of crossflow in multilateral wells, we first developed a mathematical model to predict the flow distribution in the well system, which includes the flow profile along each lateral, the flow profile from each lateral, and the total flow rate, as functions of tubinghead pressure. The multilateral well deliverability model couples the calculations of inflow to each lateral, pressure drop behavior in each lateral, and pressure drop in the main wellbore. It finds the equilibrium producing point for commingled production in multilateral wells by iteration.
Multilateral Well Deliverability Model
The well geometry considered to develop the model is illustrated in Fig.1, using a trilateral well as an example. Each lateral is assumed to be horizontal and connected to the main wellbore with a build section. We also assume in the model that the build sections are non-producing and only provide paths between the horizontal producing sections and the main wellbore. Each lateral horizontal section, each build section, and the main wellbore can have different tubulars from one another. Pressure drops in the build sections and the main wellbore are calculated using a two-phase flow correlation.
This paper proposes a method for comparing the effectiveness of cased hole gravelpacks. The method employs effective perforation tunnel permeability as the primary indicator of gravelpack effectiveness.
Completion pressure losses in cased hole gravelpacked wells are subdivided into categories and systematically reviewed to show the relative importance of alternative design features. From this information, perforation tunnel permeability is shown to be the major determinant of completion skin and flow efficiency in cased hole gravelpacked wells. The technique is shown to be useful in both low velocity (darcy) and high velocity (non-darcy) flow regimes.
Field examples are used to show how this technique can be used to diagnose design flaws and improve well performance results. These examples show that effective perforation tunnel permeabilities in cased hole gravelpacked wells are significantly lower than surface measured gravel permeabilities. This has important implications for well design and remedial candidate selection.
It is difficult to quantitatively define the quality of a cased hole gravelpack. Studies have shown that traditional quality indicators such as the skin factor or well flow efficiency of a gravelpacked well cannot be adequately correlated from reservoir to reservoir and field to field.1,2 As a result some investigators have developed a range of ranking criteria or developed their own quality indices.2-4
This paper will discuss use of perforation tunnel permeability kpt as an indicator of cased hole gravelpack quality. It will be shown that low perforation tunnel permeabilities are the main determinant of cased hole gravelpack skin factors and completion pressure losses. For the purposes of this study the perforation tunnel is defined as the linear flow tunnel stretching from the well/formation interface radius rw to the internal casing radius rei This is shown in Fig. 1.
Flow to and through a Cased Hole Gravelpacked Well
Analysis of pressure losses in cased hole gravelpacked wells necessitates a brief review of the overall flow system. For this discussion the flow system will be subdivided into the following components:
Flow to the Well: Flow to a cased hole gravelpacked well is governed by a combination of reservoir and wellbore features. In a typical reservoir/well system, the fluid moves radially through the reservoir before the flow streamlines begin to be distorted by the specifics of the well's completion geometry. For single phase flow, the first deviation from reservoir flow occurs some distance away from the well as flow begins to converge from the reservoir's constant thickness, radial streamlines to the wellbore. Partial penetration, wellbore inclination, and/or combined partial penetration/inclination effects can make themselves felt considerable distances from the well.5-7 These effects are categorized as reservoir flow convergence effects. As the flow continues to approach the well, mud filtrate damage, perforation geometry and perforation damage effects begin to be felt.8-10 These effects are categorized as well entry effects. As the name implies, well entry effects conclude when the flow has crossed the wellbore/formation rw threshold and entered the well itself.
Flow through the Well: Once flow has crossed the well/formation interface radius rw corresponding to the end of reservoir rock, it enters the wellbore. For cased hole gravelpacked completions, flow will typically move from the exterior perforations into a confined perforation tunnel which penetrates impermeable casing and cement. This perforation tunnel may be filled with formation sand, gravel, completion debris, or some combination of the three. From the perforation tunnel, flow diverges into the relatively large flow area provided by the casing/screen annulus. The flow then converges to a wire-wrapped or prepacked screen, flows through the screen, and moves to the surface. Flow through the well is described by intrawell flow effects.2,11
The flow system described above is shown schematically in Fig. 1: cased hole gravelpack flow.
Kolts, J. (Conoco Inc. ) | Joosten, M. (Conoco Inc. ) | Salama, M. (Conoco Inc. ) | Danielson, T.J. (Conoco Inc. ) | Humble, P. (Conoco Inc. ) | Belmear, C. (Conoco Inc. ) | Clapham, J. (Britannia Operator Limited ) | Tan, S. (Britannia Operator Limited ) | Keilty, D. (Britannia Operator Limited )
Brown, Lloyd D. (Conoco Inc. ) | Clapham, John (JC Engineering ) | Belmear, Clint (Conoco Inc. ) | Harris, Rob (Genesis Oil & Gas Consultants, Ltd. ) | Loudon, Allan (Nalco Exxon Energy Chemicals, L.P. ) | Maxwell, Stephen (Oilfield Microbiology Services, Ltd. ) | Stott, Jim (CAPCIS Ltd. )