Determination of the Near-Wellbore Pressure Drop for Dual Casing in Hydraulic Fracturing and Refracturing Applications

Abstract

Due to the increases in completion costs demand for production improvements, fracturing through double casing in upper reservoirs for mature wells and refracturing early stimulated wells to change the completion design, has become more and more popular. One of the most common technologies used to re-stimulate previously fracked wells, is to run a second, smaller casing or tubular inside of the existing and already perforated pipes of the completed well. The new inner and old outer casing are isolated from each other by a cement layer, which prevents any hydraulic communication between the pre-existing and new perforations, as well as between adjacent new perforations. For these smaller inner casing diameters, specially tailored and designed re-fracturing perforation systems are deployed, which can shoot casing entrance holes of very similar size through both casings, nearly independent of the phasing and still capable of creating tunnels reaching beyond the cement layer into the natural rock formation. Although discussing on the API RP-19B section VII test format has recently been initiated and many companies have started to test multiple casing scenarios and charge performance, not much is known about the complex flow through two radially aligned holes in dual casings. In the paper we will look in detail at the parameters which influence the flow, especially the Coefficient of Discharge of such a dual casing setup. We will evaluate how much the near wellbore pressure drop is affected by the hole's sizes in the first and second casing, respectively the difference between them and investigate how the cement layer is influenced by turbulences, which might build up in the annulus. The results will enhance the design and provide a better understanding of fracturing or refracturing through double casings for hydraulic fracturing specialists and both operation and services companies.

Numerical simulation of pressure loss of sliding sleeve in typical fracturing

“…Sun wantong [12] established a pressure field model of gas-liquid two-phase wellbore, which can systematically and accurately predict wellbore temperature and pressure, and analyze the influence of circulation time and geothermal gradient on wellbore pressure field. Loehken, Joern [13] studied parameters affecting flow during fracturing and the influence of casing size on pressure drop in wellbore. It can be seen that although previous researchers have carried out a lot of research on wellbore pressure field, the pressure loss after fluid passes through downhole operating tools has not been studied, and the fracturing sliding sleeve is an important tool in the fracturing process.…”