Research On The Calculation And Afterflow Correction Method Of BHP In A Rod-pumped Well In North Sa Oilfield

At present, solid measurement and build-up of fluid law are two kinds ofmethod mainly used to measure the gauge pressure in a rod-pumped well. In thesolid measurement, the pressure gauge lowered down into the bottom hole tomeasure the BHP. But the method would have restrictions in a well that does nothave a eccentric well head, and the build-up of fluid law must be taken. In NorthSa Oilfield, the number of wells measured with the build-up fluid law takes asmuch as 30% of the total number of BHP measurement wells. The calculation ofthe BHP from those materials mentioned above affects the reasonable appraisal ofthe bulk pressure and the proper implementation of the development andadjustment plan of the oilfield greatly. Therefore it has become to a matter ofconcern.The one-stage process is used mostly to calculate the BHP in the build-up offluid currently, that is to say the gradient in the bore hole is unique. Although theone-stage process method simple and fast in calculation, but the result is alwaysdifferent with the actual BHP, and has a big error. The preciseness of the pressurecalculation in the process of build-up of the fluid after the rod-pumped wellshutoff is decided by whether the pressure calculation of the gassy fluid column isaccurate. At the same time, the wellbore storage effect affects the pressure greatlyin the test of build-up of fluid. The pressure build up curve in the test has a longperiod of afterflow, and the linear portion of the radial flow is short. All these canhave a disadvantageous influence on the formation pressure calculation.Based on the analysis of all kinds of actual material and sufficient study ofphysical property distribution characteristic of the borehole fluid in build-upprocess, the three-stage physical model is established in this paper. The pressuregradient of the fluid column is recalculated and the BHP calculation is revisedaccording to the PVT parameters in the North Sa Oilfield. Combined with thecharacteristic of the early pressure curve in the build-up process and by the usingof the build-up rate of fluid and the chamber structure, the actual changing law ofthe afterflow rate is calculated, the afterflow correction and analysis theory in arod-pumped well is established, the afterflow correction method of pressurebuild-up curve in the rod-pumped well is studied in this paper, and the reservoirparameter is finally defined. The analysis and calculation is taken as to the actualdata. The results show that the calculated pressure agrees well with the actual one,and the method has the high computational precision.There are 7 chapters in this paper.Chapter1 IntroductionIn this chapter, the purposes of the pressure convert of liquid level build-upand afterflow correction is introduced, the research status is summarized, and themethod taken in this paper is stated briefly.Chapter2 Analysis of distribution character of physical property inrod-pumped hole.The curves of flow pressure vs. depth, the density of the liquid column andthe distribution of the water content in the hole is analyzed in this chapter. Theactual fluid level is calculated either. Based on the above work, the phasedistribution between the tube and the casing in rod-pumped well is given.Chapter3 Method of pressure convert of liquid level build-upThe fundamental principle of the liquid level calculation is introduced in thischapter. According to the hydraulic theory, the summation of WHP, pressureinduced by the gas and liquid column should be equal to mid-point pressure ofthe pay zone. That is PM = Pc+Pg+PL, and the calculation of PL is alsointroduced, namely PL =Po +Pow. And then the calculation Po of Po wand is statedin detail. The pressure calculation of gas column is studied, and the influence ofthe gas temperature and the compressibility on it is also discussed in this chapter.Chapter4 Well test analysis theory of afterflow correctionThe influential model of afterflow is established in this chapter. The pressureand time correction are carried out as to the influences of afterflow, and thebottom parameter is then analyzed.Chapter5 Well test analysis of fluid level build-up data in rod-pumped wellsThe correction and analysis of early time afterflow data of the build-up curveare mainly studied.1. The correction method of afterflow pressure. This is mainly used todecide Δp , that is Δp =pws ( Δt)?pwf(tp).2 The correction method of afterflow time. This is mainly used to decide theshut in time after correction, that is Δ t ′=lg Δt+aΔ1t?qsf (Δt).Chapter6 Case analysisThe correctness of the methods mentioned above is verified in two differentways. One is the analysis of pressure calculation of the liquid level build-up inrod-pumped well, and the other is the correction analysis of afterflow data inbuild-up period.Chapter7 ConclusionsThe following conclusions could be obtained based on the study above:1. In a situation that the actual pressure measurement could not beperformed, build-up of fluid law really settled the problem of formation pressurecalculation in rod-pumped wells for years, and is a unique method that could notbe substituted currently.2. As to the build-up flow law, the influences of degasification in the holewhen the pressure is lower than the saturation pressure and the influences of thevariation of mixed density and volume factor according to the pressure are takeninto account in this paper. The numerical method of iteration is adopted and thusthe accuracy of pressure calculation is enhanced.3. The convenient method of afterflow rate calculation is given in this paper.The data of afterflow rate variation could be calculated according to the variationrate of the actual liquid level in different wells, and thus realized the correction ofthe early time afterflow rate. This is a convenient method, and the inconvenientand the error caused by the description of afterflow history with mathematicfunctions are avoided.4. The correction method of early time afterflow data of the build-up curveis studied, that is the correction method of afterflow pressure and of afterflowtime. The correction method mostly used currently is improved, and the scope ofdata interpretation is enlarged. By the using of the methods given in this paper,the linear period of the radial flow in the build-up curve could be advancedobviously, and the shut off time of well could also be shortened.5. The afterflow data could be corrected exactly and immediately by theusing of pressure calculation and the correction method of early time afterflowdata of the build-up curve, and thus obtain the parameters of permeability, skinfactor, average pressure etc. The pressure calculated is in an error of 10% as to theformation average pressure.6. Case analysis indicates that the pressure calculated matches well with theactual pressure, and the formation parameters by afterflow correction aregenerally consistent with the ones that have no afterflow correction. It means thatthis method has a good application value.