Study On Borehole Stability Of Horizontal Well In Unconsolidated Sandstone

Unconsolidated sandstone reservoirs are widely distributed at home and abroad,and are distributed in almost seven continents.In China,it is mainly distributed in the Bohai Oilfield,Sichuan Oilfield,Liaohe Oilfield,Karamay Oilfield,etc.,especially in the unconsolidated sandstone reservoir of Bohai Bay,which contains abundant oil and gas resources.The use of efficient and reasonable exploration and development technology is conducive to improving the overall oil and gas production of such reservoirs in China,which has a broad prospect.However,unconsolidated sandstone is characterized by weak cementation ability,low rock strength and high porosity and permeability.In the state of high stress,the plastic deformation among unconsolidated sandstone grains is dominant.When drilling in unconsolidated sandstone reservoir,especially in horizontal section or highly deviated section of horizontal well,borehole instability such as collapse and leakage is easy to occur,which will not only increase drilling cycle and drilling cost,but also increase drilling operation risk,and bring great challenges to horizontal well drilling technology.Therefore,in-depth research on borehole stability of loose sandstone horizontal Wells will provide technical support for safe and efficient development of loose sandstone reservoirs in China.In this paper,the basic characteristics of physical properties of unconsolidated sandstone are first recognized from the microscopic level,and the mechanism of borehole instability is analyzed through the failure form of unconsolidated sandstone.Then,starting with the mechanical parameters of unconsolidated sandstone,the basic theory of formation pore pressure and ground stress,combined with the logging data,the corresponding calculation model of formation pore pressure and ground stress is determined,which provides the basic data for further research on the stability of horizontal Wells in unconsolidated sandstone.In addition,by determining the calculation models of formation rupture pressure,collapse pressure and loss pressure,a collapse and instability model of borehole wall suitable for horizontal Wells in unconsolidated sandstone reservoir is finally established,and the safety density window suitable for horizontal Wells in unconsolidated sandstone reservoir is obtained.At the same time,the risk distribution law of horizontal well wall stability and the change law of formation rupture pressure and collapse pressure with well trajectory are analyzed,which provides scientific basis for safe and efficient drilling.The field application shows that when drilling into unconsolidated sandstone reservoir,when the well inclination Angle is large and the azimuth Angle of well inclination is close to the azimuth of the maximum horizontal principal stress,the safe mud density window is narrow and the risk of collapse is high.Unconsolidated sandstone reservoir is easy to be hydrated and expanded,borehole collapse is serious,and its stability is very poor.After the hole is drilled,the natural equilibrium of the formation around the well is broken.However,the drilling fluid begins to seep into the formation under various driving forces.As the drilling fluid intrudes into the borehole,the swelling stress occurs and the formation strength begins to decrease,and the collapse pressure to maintain the borehole stability of the sandstone reservoir gradually increases.Therefore,the collapse period analysis under the coupling action of mechanics and chemistry was carried out to study the influence law of water absorption on the strength of unconsolidated sandstone,determine the calculation process of hydration collapse period,use VB program to calculate the collapse period,combined with the field application,recommend the appropriate mud density.Based on the above research results,it is not difficult to see that the properties of unconsolidated sandstone are quite different from other rocks,which makes the study on the wellbore stability of unconsolidated sandstone become a major difficulty.The research of this paper expands the field of borehole stability of loose sandstone horizontal Wells,which can not only save costs and reduce drilling risks,but also has certain practical guiding significance.