Near-wellbore fracture initiation and propagation induced by drilling fluid invasion during solid fluidization mining of submarine nature gas hydrate sediments

Drilling in a natural gas hydrate formation is challenging due to the poor consolidation of the formation and the potential evaporation of the hydrate.The unreasonable down-hole pressure of the drilling fluid can not only lead to the wellbore instability,but also change the predrilling condition of the natural gas hydrate formation,thus leading to an instable wellbore.In this paper,the integrated discrete element method(DEM)-computational fluid dynamics(CFD)work flow is developed to study the wellbore instability due to the penetration of the drilling fluid into the hydrate formation during crack propagations.The results show that the difference between in-situ stresses and overpressure directly affect the drilling fluid invasion behavior.The lower hydrate saturation leads to an easier generation of drilling fluid flow channels and the lower formation breakdown pressure.The breakdown pressure increases with the increase of hydrate saturation,this also indicates that hydrates can enhance the mechanical properties of the formation.The induced cracks are initially accompanied with higher pressure of the drilling fluid.According to the rose diagram of the fracture orientation,a wider orientation of the fracture distribution is observed at higher pressure of the invasion fluid.

Limit analysis of extended reach drilling in South China Sea

Extended reach wells （ERWs）, especially horizontal extended reach well with a high HD （horizontal displacement） to TVD （true vertical depth） ratio, represent a frontier technology and challenge the drilling limitations. Oil and gas reservoir in beaches or lakes and offshore can be effectively exploited by using extended reach drilling （ERD） technology. This paper focuses on the difficult technological problems encountered during exploiting the Liuhua 11-1 oil field in the South China Sea, China. Emphasis is on investigating the key subjects including prediction and control of open hole limit extension in offshore ERD, prediction of casing wear and its prevention and torque reduction, φ244.5mm casing running with floating collars to control drag force, and steerable drilling modes. The basic concept of limit extension in ERD is presented and the prediction method for open hole limit extension is given in this paper. A set of advanced drilling mechanics and control technology has been established and its practical results are verified by field cases. All those efforts may be significant for further investigating and practicing ERD limit theory and control technology in the future.

Strengthening shale wellbore with silica nanoparticles drilling fluid

Nanoparticles have been widely used to reduce wellbore instability problems of shale formation.In this paper,nanoparticle-containing water-based drilling fluids(WBDFs)and oil-based drilling fluids(OBDFs)were evaluated by running three new tests including spontaneous imbibition,swelling rate and acoustic transit time.Results showed that,for the WBDFs,nanoparticles leaded to higher plastic viscosity(PV)and yield point(YP),and lower API-filtration.Moreover,because pore throats of shale can be plugged by nanoparticles,imbibition amount,swelling rate,and Young's-modulus reduction of shale decreased significantly.Higher concentration of nanoparticles can induce better plugging effect.However,for the OBDFs,nanoparticles did not show these positive effects like the nano WBDFs,even leaded to some negative effects such as higher filtration and larger Young'smodulus reduction.The main reasons are that the silica nanoparticles can easily disperse in the WBDFs,and effectively prevent the filtrate invading into shale by plugging pore throats.But the same silica nanoparticles are difficult to disperse in OBDFs,and do not perform the expected functions.This study indicates that nano WBDFs have great potential to reduce the wellbore instability problems of shale formation.

An approach to improve wellbore stability in active shale formations using nanomaterials

Drilling through active shale formations has been a challenging practice in the oil and gas industry for a long period of time,given the complexity of shale structure and its interaction with Water Based Muds(WBMs).Although there have been many additives and methodologies proposed for a safe drilling through shale formations using WBMs,little success has been reported to the application of these methods once tested under different field conditions.In this paper,a new WBM formulated by nanomaterials was proposed to stabilize active shale layers during drilling.A series of rheological,density,filtration loss,bentonite dispersion and shale recovery tests were conducted on the mud samples formulated by nanosilica and Nano Glass Flakes(NGFs).The results indicated that NGF,as a cheap but effective nanomaterial,is able to significantly reduce the flirtation loss without posing any significant impacts on the density and the rheology of WBMs.It also appeared that the bentonite molecules were incapable to either hydrate or disperse in the drilling fluid system in the presence of NGFs.It seems that NGFs can stabilize clay minerals and reduce the filtration loss as remarkably efficient additive,but caution must be taken to ensure that they are properly disperse in the WBMs.