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AN IMPROVED METHOD TO DETERMINE HORIZONTAL WELL PRODUCTIVITY AT STEADY STATE

CHAPTER ONE

INTRODUCTION

BACKGROUND OF STUDY
The major purpose of a horizontal well is to enhance reservoir contact and thereby enhance well productivity. A horizontal well is drilled parallel to the reservoir bedding plane. In other words, a vertical well is drilled perpendicular to the bedding plane (see fig1.1).

Figure 1-1

If the reservoir bedding plane is vertical, then a conventional vertical well will be drilled parallel to the bedding plane and in the theoretical sense it would be horizontal well. The objective here is to intersect multiple pay zones Horizontal wells have become popular for producing oil and gas reservoirs in many regions around the world. The objectives of horizontal wells include increasing oil and gas production, turning a non-commercial oil or gas reservoir into a commercial reservoir and controlling severe coning problems. Due to the fact that horizontal wells can enhance reservoir recovery, they should be taken into consideration when planning a field development. While horizontal wells are generally more expensive to drill than vertical wells, they often reduce the total number of wells required in a reservoir development. As an increasing number of horizontal gas wells are drilled, the need for a quick and reliable method to estimate the pressure-rate behavior of these wells is important to optimize well performance and make operational decisions. A reliable and simple analytical and empirical relationship will provide engineers a technique to assess the performance of horizontal wells prior to undertaking extensive and often time-consuming simulation studies to model the well behavior. 

Inflow performance relationship (IPR) is a pressure-rate relationships used to predict performance of oil and gas wells. There is a linear relationship when the reservoir is producing at pressure above bubble point pressure i.e. when Pwf is greater or equal to bubble point pressure. A curve is obtained at Pwf less than bubble point pressure. The linear form of an IPR represents the Productivity Index (PI) which is the inverse of the slope of the IPR. Horizontal oil well IPR also depends on the flow condition that is whether it is transient, steady or pseudo-steady state flow, which is determined by reservoir boundary condition. As the use of horizontal and multilateral wells is increasing in modern exploitation strategies, inflow performance relationships for horizontal wells are needed. The objective of this work is to develop analytical equation and IPRs for horizontal oil wells in steady state conditions that are easy to apply.
Horizontal wells are high-angle wells (with an inclination of generally greater than 85ᵒ) drilled to enhance reservoir performance by placing a long wellbore section within the reservoir. There was relatively little horizontal drilling activity before 1985. A variety of configurations of drilled wells have come to be characterized as horizontal wells or drainholes. Drainholes are short length wellbores drilled pre-existing vertical wells in order to enhance production. These extend between 100 and 500 ft. in either direction. Horizontal wells on the other hand, involve the drilling of new wells and are usually 1000ft or more in length. 

The use of horizontal wells to increase the area of contacted reservoir dates back to the early 1940’s. Feasibility of creating such architecture to drain a reservoir better, has been proven for a long time but the economic viability of such a process did not establish itself until recent years. This has been largely due to titanic advances in drilling, surveying and interpretation technologies.

The following are fields around the world that have been successfully drilled and are producing through horizontal wells:

Austin Chalk Formation Texas

Spraberry Tread West Texas

Pearsall Field South Texas

Okoro Oilfield Niger Delta, Nigeria

1.2.1 APPLICATIONS OF HORIZONTAL WELL

Horizontal wells are mostly applied in the following areas;

1. To Exploit Thin Oil and Gas Zone Reservoirs: A horizontal wells reflects an increased area of contact of the well with the reservoir when compare to vertical wells in a thin pay zone. As a result of this increased area of contact, there is an increase in production from horizontal wells. 

Figure 1-2: Horizontal well following thin bed formation

2. Reduction of Coning: A horizontal well is expected to have a reduced pressure drawdown when compared to a vertical well for a similar production level. This reduced drawdown pressure is expected to delay the onset of water (gas) breakthrough. Oil recovery is expected to be high except in cases where the well intersects fractures or zones of high permeability which could result in early water (gas) breakthrough. 

Figure 1-3: use of horizontal well to minimize Water and/or Gas coning

3. To intersect fracture in a naturally fractured reservoir in order to drain them effectively.
Figure 1-4: Intersection of Fractures
4. In Enhance Oil Recovery (EOR) applications, especially in thermal EOR. Horizontal well provides a large reservoir contact area and therefore enhances injectivity of an injection well. It also helps to increase the sweep efficiency. 

1.2.2 ADVANTAGES OF HORIZONTAL WELLS

1. Improved sweep efficiency in pattern flood situations.

2. Reduction in coning. Lower drawdown for the same flow rate as vertical wells, causes stable interface movement delaying the breakthrough of unwanted fluids. 

3. Reduced fluid velocities around wellbore thereby reducing the occurrence of turbulent flow.

4. Fewer number of wells is required for field development.

5. Better access to isolated zones and exploiting gravity drainage mechanism effectively.

6. Drilling relief wells for blow out prevention.

7. Accessing untapped portion of a reservoir under constrained drilling conditions (offshore platforms).

1.2.3 DISADVANTAGES OF HORIZONTAL WELLS

1. It is not suitable for thick reservoirs.

2. Completion and Stimulation technology has not been perfected yet.

3. If natural fractures connect the aquifer or gas cap, the breakthrough of unwanted fluids can be accelerated.

4. Not suitable in low vertical permeability situations. 

5. Higher cost of drilling per well.

6. Hole problems encountered while drilling can be a serious drawback. 

1.2.4 INFLOW PERFORMANCE RELATIONSHIP (IPR)

The relation between production rate (q) and flowing wellbore pressure (pwf) over the practical range of production conditions, this relation is commonly known as inflow performance relationship (IPR). Productivity index is a measure of the ability of a well to produce. It is the ratio of the liquid flow rate to the pressure drawdown defined as below: