Geometry of Hydraulic Fractures From Modestly Deviated Wellbores

Author:

Hallam S.D.1,Last N.C.1

Affiliation:

1. BP Research Centre

Abstract

Summary Model studies of the geometry of hydraulic fractures from modestly deviatedwells revealed that the rough surfaces seen previously are formed by thecoalescence of a number of starter fractures. At higher deviation angles, thestarter fractures do not link. The overall effect is a loss of wellproductivity resulting from restricted communication length and reducedeffective aperture. This paper presents some guidelines for paper presents someguidelines for minimizing this adverse effect. Introduction Historically, most hydraulically fractured wells have been vertical. An increasing number of deviated wells now require hydraulic fracturing, in somecases because of environmental awareness and legislation, which can restrictthe number and location of drillsites. and in others because of the developmentof offshore. low-permeability fields. Productivity, from a fractured deviated well is expected to be lower than that from a fractured vertical well. Fornormal reservoir depths. the axis of an ideal vertical well lies in the preferred fracture plane, defined as the plane perpendicular to the directionof the minimum horizontal rock stress and the fracture communicates with the wellbore over the entire perforated interval The axis of a deviated well, perforated interval The axis of a deviated well, however. rarely lies in thepreferred fracture plane. and the wellbore may intersect the fracture only overa limited reservoir interval. The limited cross-sectional area of fluids toenter the wellbore from the fracture leads to high local flow rates and hence additional pressure losses and reduced productivities. particularly inhigh-rate gas productivities. particularly in high-rate gas wells The economicsof South Ravenspurn, a southern North Sea gas field, depended heavily, on effective hydraulic fracturing to achieve high productivity increases. Thefield shape and platform locations were such that all parts of the field couldbe accessed by build-and-hold wells with deviations up to about 30 deg. Suchdeviations were likely to lead to limited-entry problems. An alternative was todrill S-shaped wells to return wells to near vertical through the reservoirinterval. Doing so increases well productivity and reduces the number of wellsrequired, but also increases drilling costs and risks. The closer the wellreturns to the vertical and the higher the step-out, the higher the cost of drilling an S-shaped well. An experimental study was conducted to definemaximum inclinations through the reservoir interval as a function of well direction for test conditions, representative of South Ravenspurn. This paper describes the experiments, discusses the results, and makes recommendations forthe fracturing of deviated wells of moderate step-out. Experiments Tests were conducted on 1/4-in.-diameter model boreholes in 12-in. cubes ofplaster of Paris (hydrostone) containing silica flour to increase strength andto decrease permeability. The cubes were loaded with a permeability. The cubes were loaded with a vertical stress of 1 = 2. 1 00 psi, a maximum horizontal stress Of 2 - 2, 1 00 psi, and a minimum horizontal stress Of 3 = 700 psi. Thefield effective s were about 6.000 psi vertical and 2,000 psi minimum psivertical and 2,000 psi minimum horizontal. The intermediate stress was unknown. The preferred hydraulic fracture plane, by definition, was perpendicular to eAppendix gives full experimental details. The communication length of thefracture with the wellbore, defined here as the length of wellbore in contact with the hydraulic fracture, depends on the well azimuth, measured from thedirection and the well deviation, (Fig. 1). A wellbore lying in the preferredfracture plane has a O degs azimuth. The following tests were carried out on both open and cased bore holes: vertical well -B=0 Deg.; =90 deg. with 10, 20, and 30degs; and 0=30* with a=O, 10, 20, and 30degs. The wellbore pressure wasrecorded at 10 Hz. After testing, each sample was examined in acomputerized-tomography (CT) scanner and then opened to reveal the fracturegeometry, which was subsequently photographed. The communication length of thephotographed. The communication length of the fracture with the wellbore wasmeasured. Results and Discussion Fracture Communication With the Wellbore. Tables 1 and 2 give the results ofthe open- and cased-hole experiments, respectively. The measured communication lengths are significantly longer than the minimum theoretical length, L, for aplanar fracture perpendicular to a3. From geometrical considerations, L-d/(sin ct sin 6), where d is the wellbore diameter. JPT P. 742

Publisher

Society of Petroleum Engineers (SPE)

Subject

Strategy and Management,Energy Engineering and Power Technology,Industrial relations,Fuel Technology

同舟云学术

1.学者识别学者识别

2.学术分析学术分析

3.人才评估人才评估

"同舟云学术"是以全球学者为主线,采集、加工和组织学术论文而形成的新型学术文献查询和分析系统,可以对全球学者进行文献检索和人才价值评估。用户可以通过关注某些学科领域的顶尖人物而持续追踪该领域的学科进展和研究前沿。经过近期的数据扩容,当前同舟云学术共收录了国内外主流学术期刊6万余种,收集的期刊论文及会议论文总量共计约1.5亿篇,并以每天添加12000余篇中外论文的速度递增。我们也可以为用户提供个性化、定制化的学者数据。欢迎来电咨询!咨询电话:010-8811{复制后删除}0370

www.globalauthorid.com

TOP

Copyright © 2019-2024 北京同舟云网络信息技术有限公司
京公网安备11010802033243号  京ICP备18003416号-3