Abstract

Horizontal wells provide extended contact with the reservoir, and have unique advantages over vertical wells in many applications. As nominally horizontal wells get longer and follow more complicated trajectories, wellbore hydrodynamics becomes an important issue on well performance. In this paper, we will discuss a problem in horizontal wells - the elevation change in well trajectory and its effect on well performance. In general, a horizontal wellbore is never perfectly horizontal. The inclination angle could be a result of drilling control, or sometimes could be designed on purpose for an extremely anisotropic formation. When vertical permeability is much smaller than horizontal permeability, an undulating wellbore may be favorable to overcome the low vertical permeability. Undulation in wellbore trajectory will change the inflow distribution along the wellbore, and therefore change the wellbore performance. When two-phase flow is involved, especially gas-liquid flow, the pressure distribution in the wellbore may cause flow problems, such as trapping gas because of accumulated liquid at low spots. We use a simple analytical model to examine the flow conditions for inclined or undulated horizontal wells. The reservoir inflow is interactively coupled with the wellbore flow, and the pressure and flow rate distribution are predicted by the model.

The model then is used to analyze the development plan for the Cosmopolitan field in Alaska. The field has stress-dependent permeability, and the vertical permeability is much lower than the horizontal permeability. The results of the model explain the effect of wellbore trajectory on well performance of horizontal wells, and justify the benefits of undulating wellbores in horizontal well development in this low vertical permeability reservoir. The study showed that undulating wells in the Cosmopolitan field can help to improve well productivity.

Introduction

Horizontal wells have been used in recent years to develop oil and gas fields in many applications more efficiently than conventional vertical wells. The significant difference in drainage patterns of horizontal wells when compared with vertical wells makes it critical that the formation has to have a certain minimum vertical permeability for a horizontal well to be productive. In cases when the vertical permeability is very low, horizontal wells could lose their attraction because of lack of vertical communication. To overcome this disadvantage, a horizontal well can be designed with some undulations in the well trajectory, which will avoid the direct dependency on vertical permeability. In addition, this will further increase the contact between the well and the formation. Meanwhile, as undulations are added to a wellbore, wellbore hydrodynamics become more complicated and the effects of wellbore flow may jeopardize well productivity. The wellbore trajectory and effects on wellbore productivity and flow should be studied carefully when designing an undulating well.

The Cosmopolitan field is located in southern Cook Inlet, Alaska, USA (Fig. 1). The field was initially discovered in 1967 but never developed due to inconclusive evaluation of the reservoir. The structure was retested in 2002 with drilling the Hansen #1 wellbore and subsequently with drilling the Hansen #1A sidetrack in 20031. Initial oil in place is estimated to be over 500 million barrels. The primary producing horizons are the Hemlock and Lower Tyonek (Starichkof) Sands. The oil gravity is approximately 24 to 27°API and the oil viscosity ranges from 4 to 7 cp. Previous production tests and reservoir studies showed that the formation demonstrates stress dependent permeability, which is a function of the level of drawdown, and the vertical permeability is much lower than the horizontal permeability. This paper presents a study of potentially using undulating wells to develop the Cosmopolitan field.

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