The Art Of Productivity For Horizontal Wells

The technique of horizontal wells is widely used in oil production of China oilfield.Methods of the productivity evaluation are presented for common horizontal wells, fracturedhorizontal wells and branch horizontal wells. There are some results as below:(1) With mathematical model improvement of unsteady porous flow, new theoretical typecurves are plotted which can enlarge Arps production decline curves from leaky boundaries.(2) Two new computing methods are presented for horizontal well productivity inpressure-sensitive reservoir. Relative impact of permeability-variation-index to productivityindex is demonstrated by calculation. It is shown that productivity may reserve when theformation permeability changes in form of exponent law. Another new productivity formulaof horizontal wells is obtained in utilization by using the equivalent resistance method basedon start-up pressure gradient model. The critical spacing distance is described for differentstart-up pressure gradients which can guide exploitation plan where the horizontal well isemployed in the low permeability reservoir.(3) With asymptotic analysis to the solution of three dimension unsteady flow in circularbounded reservoir, equivalent radius models for the horizontal wells are obtained. Based onthe modes and the pressure superposition theory, productivity evaluation methods of branchedhorizontal wells spaced in different patterns are established. It is illustrated that theproductivity index and its distribution may be changed by each branch length, the branchnumber or the wedge of branches.(4) Mathematical modes of fluid transient flows are presented and solved by integraltransformation in the three-dimensional reservoir with bottom water and/or gas gap.Bottom-hole pressure formulae are obtained by the asymptotic analysis in the late period. Theequation of bottom water crest under the critical condition is presented and used to calculateboth the dimensionless water crest critical location and the horizontal well criticalproductivity. These results have an important directional significance to design the optimalproject of the reservoir with bottom water exploited by using the horizontal wells.(5) New Green-functional solutions of fluid transient flow in infinite three-dimensionalreservoir for branched horizontal wells in the plane are presented. It is claimed by calculationand analysis that bottom-hole pressure curves of the N-branched horizontal well will performthe radial flow in medium and late periods while the corresponding curves of the pressurederivative dPD/dln(tD/CD) are converged to N/2 in the corresponding condition. Dimensionlesswell length LD can determine the horizontal location of the curves and the appearance of flowregimes. The elastic productivity decline is mostly influenced by the well branch number. Asa result, the bigger the branch number is, the quicker its productivity decline.(6) A productivity formula is obtained through solving mathematical models for theunsteady porous flow of the vertical fractured wells, and then equivalent radius models arepresented. For hydraulic fractured horizontal wells, it is shown that the fracture number, thefracture length or propped fracture conductivity may modify the productivity index and itsdistribution.In the present, productivity evaluation methods of the horizontal well with complexstructure is in the studying period. Some pilot investigation on this major is presented andresults can be use to improve application impact of horizontal wells.