The geometry of a crack is a fundamental consideration when calculating leakage rates for Leak-before-Break assessments. Carrying out fluid mechanics calculations does not give any additional benefit if there is not enough information on the crack shape. To address this issue, work is being carried out under the R6 development programme to derive a model that couples fluid mechanics and solid mechanics. The aim is to combine complex crack shapes with relatively simple fluid mechanics models and compare with experimental data. Then, the model can be extended to examine various stress distributions, and give indications as to how conservative are the current models. The model is a development of the one presented in a previous PVP paper (Reference 1), and a special case of isothermal gas flow is considered, where the equations reduce to an Ordinary Differential Equation (ODE). This is solved using a Runge-Kutta integration scheme in MATHCAD. A test case is presented based on the crack geometries considered in experiments, and upon comparison with numerical results; it is clear that choosing the correct crack shape is crucial in obtaining accurate predictions of leak rate. The assumed crack openings are rectangular, diamond or elliptical. In addition to this, weld residual stress profiles are postulated, based on experience of welds in piping components. Comparing the numerical simulations with the simplified DAFTCAT model indicates that the more precise ODE method can reduce conservatism in calculation of leak rates.

A major revision of the British Standard BS7910 on “Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures” is being planned for issue in 2012. This paper provides an overview of the proposed revised guidance in relation to recommended weld residual stress profiles. As such, the paper is focussed on the proposed revised Annex Q of BS7910 which deals with residual stress distributions in as-welded joints.

Summary Extensive production testing and core analyses show that production from thelow-permeability sandstones in the Mesaverde formation at the U.S. DOEMultiwell Experiment (MWX) field laboratory is dominated by natural fractures. Stimulation data strongly suggest that damage to the narrow natural fracturessignificantly affects post-stimulation production. This paper summarizes thefield data that show evidence for production from natural fractures and fordamage to those narrow natural fractures by stimulation fluids, resulting indecreased gas production. The nature of these interactions was clarifiedthrough laboratory studies on the degradation of stimulation fluids and thepermeability damage to artificially fractured core exposed to the fluids. Evidence confirms that stimulation-fluid damage to the narrow natural fracturesrestricted the production of gas in early MWX stimulations, consistent with thepartial reversibility of fluid damage to natural fractures following along-term shut-in. A controlled breaker system was designed for use withtemperature-stable biopolymer foam. Use of this fluid system in a later MWXstimulation substantially increased gas production and reduced the evidence of damage. Introduction Three closely spaced, vertical wells were drilled through the Mesaverdeformation in the Piceance basin near Rifle, CO. The Mesaverde formation at the MWX site contains diverse depositional environments (Fig. 1): shoreline/marine, delta plain (lower, paludal environment; upper, coastal environment), fluvial, and paralic. Core was taken from all zones of production operations. Coreanalyses included a study of the reservoir properties of the matrix rock and athorough characterization of the natural fractures in the core. Results fromproduction operations showed that natural fractures dominate prestimulationproduction in all cases. Moreover, experience suggests that damage to theseapparently narrow natural fractures is a very important factor affectingpost-stimulation production. Field data indicate interactions between thestimulation fluids used and the natural fracture systems. These interactionsprobably entailed the penetration of stimulation fluids into the naturalfracture systems with a resultant decrease in permeability. The evidenceincludes the following.Observations of high treating pressures duringstimulations caused primarily by large stress contrasts between sandstones andthe abutting materials.Evidence of increased leakoff at high pressuresduring treatments (most likely into natural fractures). This appears to occurabove a threshold pressure and may result in an increase in leakoff by a factor of 50. It may be the mechanism responsible for early screenouts observed in twostimulations.Post-stimulation well-test data that clearly show the creation of a conductive fracture. We must, however, include damage to the naturalfractures that intersect the created fracture to match the low productionrates. A separate laboratory program was initiated to support the stimulationdesign. This program was expanded as a result of production problems causedlargely by the interactions of stimulation fluids with the natural fractures. Four laboratory studies were performed: analysis of stimulation fluids forpolymers and decomposition products to estimate the state of thestimulation-fluid gel in the formation; use of artificial fractures in the coreto simulate the effects of stimulation fluids on natural fractures(permeability damage and leakoff); exposure of these artificial fractures to fine-mesh sand; and design of a breaker system for use with a biogel polymer tominimize damage to the natural fracture systems. The three objectives of thispaper are to establish through MWX field and laboratory data the importance of interactions between natural fractures and stimulation-fluid systems, todetermine the interaction between the natural fracture and thestimulation-fluid systems by describing laboratory studies on the degradation of the fluid systems and permeability damage of artificial fractures ex posedto stimulation fluids, and to describe a stimulation-fluid system developed foruse in tight sandstones containing narrow natural fractures. MWX Stimulations and Fluid Systems Productions and stimulations were conducted during 1983–87 in each majorlenticular depositional environment: in paludal Zones 3 and 4 together; in thecoastal yellow sand, and in each of the fluvial Sands B, C, and E (Table 1). Minifractures were conducted to determine stimulation parameters of fractureclosure, fracture-height growth, and leakoff for use in the final design of theassociated propped stimulations. The minifractures were designed to beapproximately pad volumes. Propped stimulations were to evaluate fracturingbehavior in lenticular reservoirs and to increase production. Water-basedgelled hydroxypropyl guar (HPG) system with a methanol prepad and a 75% qualitynitrogen foam with gelled biopolymer (xanthan gum) (20 lbm/1,000 gal [2.4kg/m3] in the liquid phase) were used. Table 2 summarizes the various treatmentcompositions. All fluid systems contained bactericide and surfactant (see Refs.1 through 10 for further details). HPG Stimulation-Fluid Systems. HPG gel with methanol prepads was used in thepaludal-zone stimulation operations. Two minifractures and a proppedstimulation were conducted in the paludal interval (Table 1). The minifracturefluids contained an HPG gel with an oxidizer breaker concentration of 1 to 2lbm/1,000 gal [0.12 to 0.24 kg/m3] (Table 2). The fluid system contained a claystabilizer. The stimulation fluid contained a Ti crosslinked HPG gel (Table 2). Breaker concentration was low -- 0.25 to 0.50 lbm/1,000 gal [0.03 to 0.06kg/m3]. The breaker was used in only the last two stages (in about 30% of thefluid pumped) because of the possibility that the prevailing formationtemperatures (210F [99C]) would cause the stimulation fluid to lose viscosityand drop the proppant. Post-operation production was less than the pretreatmentproduction, suggesting significant formation damage. Because of productionproblems after the propped paludal zone stimulation, a small-volume, highlyoxidizing breaker treatment was attempted (Table 2). This treatment, whichremoved a possible gel block in the sandpack, was conducted below the fractureclosure pressure to prevent any natural fractures from opening. There was noobvious production increase as a result of the treatment; in fact, this highlyoxidizing treatment may have caused some formation damage. 3 No production wasrealized until a modified packer assembly was installed to facilitateproduction of returned fluids.

A ratio of shoulder to gauge displacements (S2G) is calculated for three different fatigue specimens in a pressurized water environment. This ratio needs to be known beforehand to determine the applied shoulder displacements during the experiment that would result in the desired strain amplitude in the gauge section. Significant impact of both the applied constitutive law and specimen geometry on the S2G is observed. The calculation using the fully elastic constitutive law results in the highest S2G values and compares very well with the analytical values. However, this approach disregards the plastic deformation within the specimens that mostly develops in the gauge section. Using the constitutive laws derived from actual fatigue curves captures the material behaviour under cyclic loading better and results in lower S2G values compared to the ones obtained with the fully elastic constitutive law. Calculating S2G values using elastic–plastic constitutive law based on the monotonic uniaxial tensile test should be avoided as they are significantly lower compared to the ones computed with elastic–plastic laws derived from hysteresis loops at half-life.

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