Research On Ordovician Buried Hill Reservoir Characteristics And Fracturing Working Fluid System Optimization In Jizhong Area

Ordovician reservoirs in the Centre Hebei are characterized by disparate fracture development,lack of homogeneity and short acid-etched distance.In addition,normal aqueous fracturing can hardly meet the requirements in reforming the complex reservoirs.Under such circumstances,researches on fracturing fluid in responding to the characteristics of the special Ordovician reservoirs are needed.Based on drill core,slices,experimental data and other resources,this paper systematically researched on the petrology characteristics,reservoir properties,reservoir water sensitivity,acid sensitivity and stress sensitivity of Ordovician buried hill reservoirs in Huabei Oilfield and evaluated on the reservoirs in general.Most of the rocks in the Ordovician buried hill in the central Hebei are limestone and dolomite.Calcite,some dolomite and a small quantity of anhydrite,flint and clay clay minerals are the main components of the rocks.Inter-granular pores,dissolution pores,karst caves and structural joints are the major types of the reservoir space.There are three categories of the reservoir spaces:vast cavern-large crack;tiny crack-small hole;the combination of crack and hole.The reservoirs are lack of physical property with an average permeability of 0.625mD.and with poor pore structure.Generally speaking,these reservoirs are with low permeability.Compaction,silting,corrosion,dolomitization and recrystallization are the major types of diagenesis,within which the latter three methods can enlarge the space and increase the porosity.The reservoir type is mainly fractured karst cave type.The water sensitivity of the Ordovician buried hill reservoirs is below the average in general.The water based drilling fluid can be used as the working fluid;Acid filling can help increase seepage force.The acid fluid can improve reservoir permeability greatly after it entered the layer.Acid fracturing can be used in this case owing to the absence of acid sensitivity.However,these measures should be analyzed adjusted in time in application because of high crack sensitivity and the great impacts of changing formation pressure on fracture permeability.Based on the characteristics of the Ordovician reservoirs and assessment on the application of common fracture fluids,it can be concluded that the acid corrosion rate of the Ordovician reservoirs has exceeded 78%,which is benefit for acid-fracturing.More importantly,clean acid fracturing working fluid reacts to acid-rock at a slower rate than other types of fracture fluids.It can better increase the depth of penetration and extend the operating distance of acid-rock reaction.As a consequence,clean acid fracturing working fluid is preferred in the acid fracturing.Then based on the performance evaluation of clean acid fracturing working fluid,it can be concluded that this type of fracturing fluid also have advantages in the aspects of residue,reflow performance,viscosity and homogeneous acid-etching.As a consequence,clean acid fracturing working fluid is selected in the reservoir reform.Clean acid fracturing working fluid performed well in reforming the reservoir while constructing the Well Antan 1.It helped provide an access to an industrial hydrocarbon flow after drilling an exploratory.According to statistics generated by earthquake monitoring after the fracturing,the crack is 572m in length,372 in width and 129m in height,which has met the requirements of construction and design.The results indicated that clean acid fracturing working fluid can be used in reforming the Ordovician buried hills reservoirs.