A Study of Fracture Initiation Pressures in Cemented Cased-Hole Wells WithoutPerforations

摘要

A new method of completing multiple-layer tight gas wells isbeing investigated. The main concept is to place sliding sleevevalves in the casing string and complete the well with normalcementing operations. The sliding sleeves would then beopened one at a time to fracture layers independently withoutperforating. The possibility of high fracture initiationpressures is identified as the main risk with this approach.This paper will discuss the theoretical and experimental studythat was conducted to assess the viability of the cementedsliding sleeve concept by attempting to minimize and predictfracture initiation pressures.Finite Element Analysis (FEA) was conducted to estimate thestresses in the cement and formation near the wellbore withsliding sleeve. FEA was used to adjust valve parameters thatincreased tensile stress in the cement and formation.Unstressed cement tests were then conducted on a variety ofsliding sleeve valve shapes to verify the FEA study and toselect the best valve shape.Openhole and perforated casing fracture initiation pressureswere calculated as a function of rock properties and far fieldstresses on the rock. An openhole condition was consideredthe best approximation to the opened sliding sleeve valve inregards to fracture initiation.Full-scale stress frame tests were conducted using sandstoneblocks with far field stress applied. The base case was set upusing 4-1/2-in diameter casing cemented in one block and thenperforated in the preferred fracture plane. Another sandstoneblock had a sliding sleeve valve cemented in place. Water wasused to fracture these blocks and the fracture initiationpressures were measured. Good agreement between predictions and measurements was obtained, and the resultsindicated that high fracture initiation pressure is unlikely to bean issue with this completion method.