Optimization of Marginal and Partially Depleted Oil Fields
Authors: Nelson Jack R. 
Published: December 2007

Dr. Jack R. Nelson, SPE

Abstract
Increased recovery from existing, marginal and partially depleted oil fields has been the objective of many operators over the years. Conventional step out drilling, redrilling, and reentering are often options to consider in these cases. Improvement in recoverable production may be obtained, although economics may be questionable in many cases.
Horizontal drilling technology has been widely and successfully applied around the world for the recovery of heavy oil, light oil and gas. Impressive technical successes have been achieved in horizontal technology through a variety of improvements in tools, and experience gained over the past several years.
Utilizing horizontal drilling technology in developing and increasing productivity of marginal fields has improved economics in most instances. Also it has provided solutions to the recovery of oil from inaccessible areas in reservoirs such as undrained fault blocks. After abandoning old wells, the existing wellbores are re-entered and horizontally redrilled through the pay zone.
The primary objective of a horizontal well is to increase the wellbore contact with one or more reservoirs. Often this contact will be limited to a specific interval in the reservoir. There are factors and limitations that determine the success of horizontal wells.
This paper will discuss the selection of reservoir candidates, planning options, re-entry drilling, and well completion of the horizontal candidates.

Introduction
In recent years, horizontal drilling techniques have been successfully used worldwide in a variety of situations. The impressive technological and economic success achieved has resulted from new and improved tools, better methodology, and a wide background of experience that has been developed over time.
Horizontal drilling is being commonly used today as a technique for improving recovery and productivity from marginal or partially depleted fields because it offers the benefit of greatly improved drainage patterns from a minimum of well bores. This application may take the form of re-entering existing well bores and drilling a horizontal hole through a window milled in the casing or new wells may be drilled as necessary. Variations in application are many. The horizontal well bores may be used to improve existing well patterns or, to reach inaccessible areas of the fields or, to permit economic production levels from reservoirs in advance stages of depletion which are subject to water or gas coning.

Benefits of Horizontal "Re-Entry" Drilling
Horizontal wells must be considered today as an attractive way of developing reserves. Drilling horizontally is not an objective itself, the objective is increased production and reserves. A comparison can be made between the productivity of a horizontal vs. a vertical well using different parameters such as spacing, permeability and length of horizontal sections to more easily visualize the capability of horizontal wells for improving production and increasing reserves. These comparisons are based on ideal considerations seldom encountered in real reservoirs but they serve to illustrate the advantages of horizontal drilling.
The comparison will start with spacing (Fig. 1) which is based on the calculation formula of Dr. Joshi. Assuming that the net pay is 50 ft., the horizontal permeability is equal to the vertical permeability, the wellbores have radii of 3.5 in., and that the horizontal sections are 1600 ft., the productivity of each horizontal well is almost 10 times higher than the vertical well for a vertical-well-drainage area of 40 acres.
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