Evaluation And Application Of Three-dimensional Fracability In Tight Reservoirs

Tight reservoirs are characterized by low porosity & permeability and developed natural fractures,and hence,they need to be fractured to develop.Fracability is the capability of the reservoirs to be fracture stimulated effectively and is the comprehensive reflection of geology and reservoir properties during fracturing.So far,most researches of fracability focus on shale rock,whose main purpose is to search the area named the fracturing dessert on the plane.However,the existing methods cannot meet the requirements which help select the fracturing well and determine the fracturing internal at the same time when fracturing in the oilfield.This paper overcomes limitations from the conventional evaluation methods of fracability which utilize the indoor experiments & log data to obtain rock mechanical factors and then characterize the fracability in tight reservoirs.In this method,both seismic & log data are used to characterize the geological & mechanical properties of reservoirs,and then to establish the fracability model,which can not only select the fracturing well on the plane and also select the fracturing interval along the longitudinal direction.Finally,the method is applied to an igneous rock reservoir in Xinjiang Oil field.During the application,it conducts the description of reservoir geology and mechanical properties,optimizes the influence factors of fracability,and evaluate the value of fracablity of each well by fuzzy comprehensive judgement.The main work is as below:1.Study of the geology and mechanical properties of igneous rock reservoirsThe Log Constraint Impedance Inversion Technique is employed to study the geology and mechanical properties of igneous rock reservoirs.During the process,We obtain the plane distribution of P-wave impedance within the P1f1 layer,invert the rocks acoustic velocity?density and other data,compute the three-dimensional data volume of rock mechanical parameters in the working area.The results show that the south part of T8 working area has more hard rocks with larger value of elasticity modulus while more soft rocks with smaller value of elasticity modulus in the north area;In addition,the integrated analysis of seismic curve & deviator strain density indicate that natural fracture system is well developed in the working area.2.Optimization of the influence factors of fracability in igneous rock reservoirsAccording to the scientific definition of fracability,both of the reservoir geological and rock mechanical properties are taken into consideration,and hence,all influence factors,e.g.,diagenesis?anisotropy between layers?geo-stress?potential of reactivation of natural fracture and rock brittleness are analyzed.The results show that the stress difference in T8 working area is about 40~48MPa;when the fluid pressure coefficient reaches 2.06,most natural fractures can be reactivated and connected;the calculation results reveal that the average brittleness index of Well U2 and SE13 reach up to 90/100.3.Establishment of evaluation method of fracability by fuzzy comprehensive judgementBased on the complexity of influence factors of the working area,fuzzy comprehensive judgement is employed to establish the fracability model.Firstly,the influence factor set,including diagenesis?anisotropy between layers?geo-stress?potential of reactivation of natural fracture?rock brittleness,is built.Then,partial membership function is used to determine the single factor assessment matrix.The judging results indicate that Well SE13 & S102 have good fracability(Level I,assessment value higher than 0.7),Well Y8 has bad fracability(Level III,assessment value lower than 0.4),the rest four wells have general fracability(Level II,assessment value between 0.4 and 0.7).