Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure （FIP） either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near- wellbore region, rather than by mudcake strength. Fluid loss pressure （FLP） should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

A NEW MODEL FOR CALCULATING PROPAGATION PRESSURE AND INITIATION PRESSURE OF PIPELINES WITH A LOCAL DENT

An analytical model of a ring with six yield hinges and two deformable arc segments is presented for. the prediction of the buckle propagation pressure and initiation pressure in offshore pipelines. The configuration of a fully collapsed ring is considered as a real dumbbell shape with a line touch between two 'bells', instead of the dumbbell shape with a point touch of two diametrically opposite points. Calculations are performed assuming that the dominant effect on the plastic energy dissipation has the circumferential bending mode. For the linear strain-hardening materials it is found that theoretical predictions based on the above model for both propagation pressure and initiation nressure are in good agreement with experimental results of Kyriakides et al.

Tuning of band-gap of phononic crystals with initial confining pressure

The mechanism of the shift of the band-gap in phononic crystal （PC） with different initial confining pressures is studied experimentally and numerically. The experimental results and numerical analysis simultaneously indicate that the confining pressure can efficiently tune the location in and the width of the band-gap. The present work provides a basis for tuning the band-gap of phononic crystal in engineering applications.

Investigation on the Determination of Initial Shock Pressure at Near Interface Field of TNT Charge and Water for Underwater Explosion

There are few report on the directly measurement of the initial shock pressure of explosive charge at its interface of water for underwater explosion. The special technologies have been taken to the measurement system with manganin piezoresistive gauge （PRG） in order to measure the initial shock pressure at the interface and its near field of TNT chare and water. The free-holding PRG film gauge can directly determine the shock peak pressure at the interface and near field of TNT charge up to 12.85 GPa, which is satisfying for the good agreement to the 12.97 GPa with one dimensional theoretical analysis and 12.86 GPa with numerical simulation. The maximum discrepancy is 0.93 %. The results show that it is precise and reliable to determine the initial shock pressure of underwater explosion charge with the PRG technology.

Affection mechanism research of initiation crack pressure of perforation parameters of horizontal well

Horizontal wells show better affect and higher success rate in low water ratio cement,complex fracture zone,crevice and heavy oil blocks,it is the main measures to expand control area of a single well.Hydraulic fracturing technology is the most financial way to improve the penetration of the reservoir to increase the production.However,compare with the vertical wells,the fracture of Horizontal wells are more complex,and lead to the initiation crack pressure is much higher than vertical wells.In this paper,defined the crack judging basis,and established the finite element model which could compute the initial crack pressure,to research the affection mechanism of perforation azimuth angle,density,diameter and depth,to provide references of perforation project's design and optimize.The research of this paper has significances on further understanding the affection mechanism of perforation parameters.

Experimental study on detonation parameters and cellular structures of fuel cloud

In this paper, detonation parameters of fuel cloud, such as propylene oxide （PO）, isopropyl nitrate （IPN）, hex- ane, 90# oil and decane were measured in a self-designed and constructed vertical shock tube. Results show that the deto- nation pressure and velocity of PO increase to a peak value and then decrease smoothly with increasing equivalence ra- tio. Several nitrate sensitizers were added into PO to make fuel mixtures, and test results indicated that the additives can efficiently enhance detonation velocity and pressure of fuel cloud and one type of additive n-propyl nitrate （NPN） played the best in the improvement. The critical initiation energy that directly initiated detonation of all the test liquid fuel clouds showed a U-shape curve relationship with equiva- lence ratios. The optimum concentration lies on the rich-fuel side （;b 〉 1）. The critical initiation energy is closely related to molecular structure and volatility of fuels. IPN and PO have similar critical values while that of alkanes are larger. Detonation cell sizes of PO were respectively investigated at 25;C, 35;C and 50;C with smoked foil technique. The cell width shows a U-shape curve relationship with equivalence ratios at all temperatures. The minimal cell width also lies on the rich-fuel side （;b 〉 1）. The cell width of PO vapor is slightly larger than that of PO cloud. Therefore, the deto- nation reaction of PO at normal temperature is controlled by gas phase reaction.

Failure Mechanism of O-Ring Seals under Extreme Operating Conditions

In this paper the behavior of an O-ring made of NBR rubber was investigated under extreme conditions. The effect of the extreme initial compression, operating pressure and extreme temperature conditions were examined. The rubber material was tested in simple tension, pure shear and equibiaxial tension modes complemented with a Dynamic Mechanical Thermal Analysis (DMTA) to capture the viscoelastic behavior of the material. For the investigation, a large-strain viscoelastic material model was developed by the authors, to take into account the large deformations caused by extreme conditions. Insufficient space during installation causes extreme initial compression consequently leading the material to crack on the contacting outer surfaces. It was found that the excessive strain and friction induced shear stress contributes primarily to this phenomenon. Extreme operating pressure causes the seal to penetrate into the gap between the shaft and the housing. This behavior damages the material and cracks appear on the seal. High strain areas were found in the proximity of the gap in the material. The analysis of the extreme operating temperature showed that during cooling the O-ring can completely loose its ability to seal at -70°C. There are three contributing factors: the speed of cooling, the temperature and the coefficient of thermal expansion.