Abstract

Relatively heavy crude production has historically been a problem in vertical completions in mature and new development fields in the Gulf of Mexico. Horizontal wells have been utilized to minimize water coning in potential completions that are close to the oil/water contact. However, horizontal completion costs are typically 60% more than vertical completions, consequently well and fluid design are critical to minimizing costs associated with drilling (fluid losses, wellbore stability, etc.).In addition to the engineering challenges for horizontal drilling, formulating a reservoir drill-in fluid (RDF) that minimizes invasion and fluid losses to the reservoir, provides compatibility between the reservoir fluids and the RDF fluid, and readily cleans-up is critical to success.

This paper will demonstrate the use of sidewall and conventional core material and subsequent petrophysical techniques to design a compatible RDF system. Core material from offset wells and previous vertical wells in several sands were utilized to develop bridging-material blends, determine sorting, texture, and particle and pore size.The authors discuss the advantages and disadvantages of using this type of material as well as that of a relatively new computer technique that allows a pore-size distribution to be rapidly calculated from core material.Another technique utilizes the rock attributes and log information in conjunction with a rock catalog to facilitate rapid determination from analogs. This type of data, in turn, can ultimately be utilized to design an RDF system.As such, this method will also be described and contrasted with traditional methods.

Finally, case histories from several recently drilled horizontal wells that incorporated the subsequent RDF bridging-solids design using the aforementioned techniques will be presented. Fluid losses and solids loading will be examined with respect to the RDF designs.

Introduction

This paper addresses the uncertainty involved in designing a reservoir drill-in fluid (RDF) for a horizontal, deviated, or extended-reach well and its inherent bridging-solids formulation. The methods that are utilized to determine an appropriate bridging solids distribution vary.[1–4] Often a petrophysical method is employed and involves the utilization of optical and/or electron microscopes, core permeameters, and/or porosimeters to analyze core material. Other data such as grain size, sorting, pore type, mineralogy, and texture can be derived from these instruments. However, the data reliability is sample dependent. The continued development of computer-associated software has helped to mitigate operator error.

In many cases, old sidewall and/or conventional core splits are stored after retrieving from initial exploration and even ongoing development wells. However, these samples are subject to degrees of skepticism regarding their current or future value.In addition to analytical techniques, this paper outlines a technical discussion that addresses the sample quality uncertainty, and demonstrates that old percussion sidewall core material can provide as much rock fabric information as pristine conventional core.

The petrophysical techniques that will be discussed include, petrographic image analysis (PIA), mercury intrusion porosimetry (MIP), rock comparator, and traditional methods that include Scanning Electron Microscopy (SEM) and optical analyses that utilize previously prepared coated/uncoated and polished epoxy impregnated thin section samples, respectively. A myriad of data[5–11] and physical parameters can be derived from these techniques. For the purposes of this paper, the emphasis will focus on the ability to derive pore-size information from these available core samples. The pore information, in turn, can be utilized to design the bridging particles for a RDF system.[12]A properly designed particle-size distribution, in turn, provides many advantages. These include the minimization of fluid and/or filtrate into the producing reservoir, deposition of a thin filter cake that, in part, enhances clean-up and return permeability.[13]

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