Yunhu Lu

A Mechanical Model of Borehole Stability for Weak Plane Formation Under Porous Flow

Abstract Based on influence of porous flow on weak plane model, the authors established a mechanical model of borehole stability for weak plane formation under porous flow and analyzed effect of weak plane on borehole stability under porous flow. The results indicated that porous flow decreased strength of weak plane, enlarged the affecting domains of weak plane for rock mass strength, and worsened borehole instability in weak plane formations. With porous flow increasing, water content of weak plane increases. For the weak plane with DIP < 10°, the borehole is stable; for 10° < DIP < 30°, the borehole is likely unstable when azimuth is close to the direction of maximum stress, and the larger the dip, the worse the borehole stability; for DIP > 30°, it is opposite to the situation of 10° < DIP < 30°. While weak plane formation is next to be saturated, the minimum drilling fluid density for borehole stability does not change with weak plane azimuth and borehole stability is the worst. The mechanical model of borehole stability for weak plane formation under porous flow is applied to a well in Tarim Basin piedmont.

Influence of Porous Flow on Wellbore Stability for an Inclined Well With Weak Plane Formation

The criterion of slip condition and wellbore instability are strongly affected by porous flow in weak plane formation, which leads to wellbore instability in the formation with weak planes. Considering influence of porous flow, a mechanical model of wellbore stability was established for inclined well with weak planes under porous flow, and the influence of various hole deviation angle, hole deviation azimuth, and weak plane occurrence on wellbore stability were analyzed. The results show that porous flow has worsened wellbore instability, and inclined wellbore stability is highly dependent on the weak plane occurrence, hole deviation azimuth, and deviation angle under porous flow. The mechanical model of wellbore stability for inclined well with weak planes under porous flow is applied to sidetracked hole in an oilfield of western China.

Analysis of the vertical borehole stability in anisotropic rock formations

The objectively existing in situ stress field and the physical mechanical properties of rock are closely related to the borehole stability in petroleum engineering. However, in present engineering design, rock mass is simply treated as isotropic material. This method may be acceptable for shallow rock engineering, but for deep rock engineering, with the increase of drilling depth, the anisotropic properties of rock mass become stronger and should be considered. In the past, accurate methods to predict critical fracturing or collapse pressures were unavailable. Simple isotropic stress equations have been used to some extent, but these have failed to take into account real rock properties that are clearly anisotropic. On the basis of some rock testing experiments, the vertical borehole stability in transversely isotropic media was the main focus of this study. By solving the stress distribution on the borehole wall, a new vertical borehole stability model was established. The results obtained in this study showed that the anisotropy of the rock and the horizontal stress ratio greatly affect the stress distribution and the failure plane of vertical wellbores. Neglecting this effect can lead to errors in stability predictions. Therefore, it was seen that the effect of the rock anisotropy is of practical importance in the life of a well since it can avoid borehole instability issues.

Identification of leak zone pre-drilling based on fuzzy control

The key factor of plugging operation is how to identify leak zone as well as the difficult point for drilling operation in complex formation. Drilling fluids leakage and reservoir contamination frequently occurred due to the traditional method of identification of leak zone that lost circulation had already happened before detected. In this paper, the difficult problem of technology in identification of leak zone before drilling is solved by using fuzzy control theory. According to analyzing relevant parameters of lost circulation, the relationship between Euclid approach degree and lost circulation is established by using fuzzy relationship of lost circulation parameters. The field application indicated that this method is more accurate and workable in forecasting and practicing whereas the cost is effective. Above all, good results have been achieved in predicting the risk of leak zone by introducing this method.

The Development and Application of an Environmentally Friendly Encapsulator EBA-20

Abstract At present, the encapsulators of drilling fluids used in domestic oilfield are mainly organic polymer compounds. These encapsulators show good performance in their own drilling fluid systems. However, they also demonstrate some defects, such as narrow range of applicability, unsatisfactory temperature resistance and salt tolerance, incapability to meet the requirement of corresponding national standards for environmental protection, and so on. Using biopolymer as the main monomer and mixing with nonionic, anion, and cationic monomer, the authors develop an environmental friendly encapsulator EBA-20 with temperature resistance and salt tolerance by aqueous solution polymerization method. Laboratory test and field application show that the synthetic encapsulator EBA-20 can improve the viscosity and yield value of drilling fluid and reduce the fluid loss of drilling fluid. Meanwhile, its temperature resistance and salt tolerance are also better. The contents of main heavy metals, such as chromium and mercury, are under the standard level of GB4284-84; the value of acute biotoxicity test EC50 is greater than 30,000 mg/L, which has little influence on environment; the value of biodegradability BC is 20.1, which shows that EBA-20 is biodegradable and is useful for drilling fluid system to improve its environmental protection performance. Formulation #1, which takes environmentally friendly encapsulator EBA-20 as marine drilling fluid system, meets the requirements of drilling operation and has an excellent environmental protection performance. It could reduce environmental pollution to the operating area in the marine.

Experimental study on the performance of sand control screens for gas wells

Based on the sand control mechanism, similarity criterion and factors affecting the stability of borehole plastic region, a large-scale laboratory test apparatus for simulating the performance of sand control screens for gas wells was developed. A series of sand control simulation experiments were performed. Based on the particle size analysis of formation sand and geological conditions, parameters of sand control screen, such as sand retention efficiency, were investigated. By simulating formation conditions, such as pressure gradient and gas flow rate, the performance of several types of screens, including slotted screen, wire wrapped screen and premium composite screen that are commonly used in oilfield, was evaluated. Experimental results were discussed and premium composite screen was found to be most efficient for sand control among all the tested screens. This study provides a sound experimental method for evaluating the performance of sand control screens for gas wells.

The Solubility and Rheology of Live Oils Saturated by Different Alkane Gases

Using self-developed equipments, the solubility and rheology of Kelamayi live oils saturated by different alkane gases were investigated at pressures 0–2 MPa. The gas-oil ratio, Rs of the live oils increases greatly with increasing saturation pressure while changes little with saturation temperature. The pour point, abnormal point, viscosity/equilibrium viscosity, and yield stress of the live oils decrease significantly with increasing saturation pressure, meaning that the rheological properties of the live oils improve greatly by gas dissolving. The ability of the three alkane gases to improve the solubility and rheology of the live oils could be described as LPG>ethane>NG.

Multilayer Pressure Containment Model and its Application in Deep Well Fractured Formation

List of Symbols H Thickness of the three layers h Half the thickness of the middle layer lU Half the height from the center of the middle formation to the upper plugging position SU Stress difference between the upper and middle layers gin situ(y) In situ stress gradient gv|y| Vertical pressure drop gradient of fluid gfluid(y) Fluid gravity gradient rh Minimum horizontal in situ stress rmain Basic part of the minimum horizontal in situ stress lD Half the height from the center of the middle layer to the lower plugging position SD Stress difference between the lower and middle layers Pf Fluid column pressure Pmain Main part of the fluid column pressure yo Difference between the middle of the middle layer and the middle of H Wf Width at the fracture entrance E Elastic modulus V Poisson’s ratio K Stress intensity factor rin situ Minimum horizontal in situ stress