Swami, Vivek (CGG) | Tavares, Julio (CGG) | Pandey, Vishnu (CGG) | Nekrasova, Tatyana (CGG) | Cook, Dan (Bravo Natural Resources) | Moncayo, Jose (Bravo Natural Resources) | Yale, David (Yale Geomechanics Consulting)
In this study, a state-of-the-art seismic driven 3D geological model was built and calibrated to a petrophysical and geomechanical analysis, 1D-MEM (Mechanical Earth Model), on chosen wells within the Arkoma Basin of Oklahoma. The well information utilized in this study included basic wireline logs and core analysis, including XRD (X-Ray diffraction) data. The traditional petrophysical analysis was augmented with advanced rock physics and statistical techniques to generate the necessary logs. Hydrostatic, overburden and pore pressures were calculated with a petrophysical evaluation model. The 1D-MEMs were based on the Eaton/Olson/Blanton approach with the HTI (Horizontal Transverse Anisotropy) assumption. The 1D-MEMs were calibrated to laboratory data (triaxial tests) and field observations (mud logs, wellbore failure, frac pressures). Therefore, a very good confidence was achieved on Biot's coefficient, tectonic components, anisotropy and dynamic to static conversion factors for Young's Modulus and Poisson's Ratio. Seismic inversions were performed in different time windows and merged to generate high resolution P- and S-Impedance attributes from surface down to the target interval after careful AVO compliant gather preconditioning. A density volume estimate was calibrated to well data, accounting for different geological formations, to decouple P- and S-Wave components as a 3D volume, as well as dynamic Young's modulus (E) and Poisson's ratio (PR). Dynamic E and PR were converted to static parameters using results from 1D-MEMs; and 3D models of Biot's coefficient (α) and tectonic components were built to compute 3D fracture pressure volumes calibrated to well data. The final products were seismic-driven 3D pore pressure and fracture pressure calibrated to 1D-MEMs. The correlation between measured/estimated well logs and corresponding seismic-derived pseudo logs was more than 80%, which indicates good quality of seismic inversion results and hence 3D-MEM. Also, stress barriers, anisotropy, and brittleness indices were calculated on well scale which would help to identify best zones to place hydraulic fractures. The 3D geological model will aid in identifying sweet-spots and optimizing hydraulic fractures.
The Rosenwald pool, Okfuskee County, Okla., was discovered in June, 1949. Production is from the Union Valley limestone and Cromwell sand, both of Pennsylvanian age, at an average depth of about 3,450 ft. The Union Valley lies conformably on the Cromwell with the two formations forming a common reservoir. The pool is a stratigraphic trap with the limits defined by a pinchout of the sand and a loss of permeability in the lime. The pool as fully developed consisted of 31 producers drilled on 10-acre spacing.
All wells flowed upon completion, with initial productions ranging from 40 BOPD for edge wells to as much as 1,600 BOPD where good Cromwell sand development was encountered. Initially only a few feet of sand penetration was taken, for fear of encountering bottom water.
Early in the life of the reservoir, performance calculations were made using Muskat's differential form of the material balance equation for dissolved gas drive reservoirs, in an attempt to predict the future production history of the pool. A kg/ko curve for the performance calculations was constructed from data of other pools which have produced from Cromwell or sands similar in texture to the Cromwell. Subsequently the reservoir has been essentially depleted by primary means, enabling a comparison of the actual performance with that predicted. Actual recovery was 20 per cent of the original oil in place as compared to the prediction of 26½ per cent. Laboratory-determined relative permeability ratios and the assumed ratios used in the performance calculations are compared with those calculated from the field performance.
By the end of 1953 most of the wells had become marginal producers and the pool was unitized and is presently being waterflooded.
The Rosenwald pool has afforded an excellent opportunity to analyze the performance of a solution gas drive reservoir. This relatively small pool was drilled on 10-acre spacing and had an over-all primary producing life of 41½ years. Care was taken throughout the life of the pool to obtain adequate production and engineering data to enable good comparisons of the actual performance with theoretical concepts.
When an oil operator becomes a party to a proration agreement he may wonder,with good cause, whether production prorated today is merely deferred untiltomorrow or whether oil might be lost. Various types of evidence obtained fromproduction records when studied quantitatively are of great assistance inarriving at definite conclusions. A study of individual examples cited in thispaper leads towards some solution of the problem, particularly when they areconsidered in the aggregate.
Many production curves on various pay horizons were studied. A disappointinglylarge number of these were of contributory value, but inconclusive. However,there were enough curves of similar type from the Wilcox sand at Earlsboro,Bowlegs, Seminole City, the Cromwell horizon at Little River, and the Simpsondolomite at Valley Center, Kansas, from which reasonably definite conclusionswere drawn.
Assuming that deferred production is later made up with no increase in ultimaterecovery, a curve (Fig. 1) shows the normal flowing and pumping life of acomposite well, constructed by the family curve method. The initial productionof the well shown is 1700 bb1. daily and the allowed production is 25 per cent.On this basis the average daily allowed rate for the first month is 425 bbl.,or the area BCDE, which is equivalent to the area AFDG under the normal declinecurve. The potential production for the second month is 1500 bb1. and theallowed production 375 bb1. daily By this procedure a potential curve for eachsucceeding month is obtained (points A, H, I, J and K). If the well is openedit should produce somewhere near its potential curve for a short period andthen decline at a more rapid rate than the potential curve. This theoreticalanalysis is basic. Actual examples exhibiting all points raised by the idealcurve are rare.
When a well strikes an oil-bearing layer, the oil has a pressure which isgenerally sufficient to enable it to rise to near the surface (sometimes abovethe surface). As soon as a well begins to produce, however, the liquid movesthrough the pores of the reservoir bed and the pressure in the well becomesmuch lower than the pressure originally prevailing there. At some distance fromthe well, however, the pressure in the reservoir bed remains unaltered; thusthe pressure of the oil has not only to lift the oil, but also to overcome thefriction resistance in the pores. The fact that so many oil wells are gushersis a consequence of the energy accumulated in the gas.
In gushing the well acts as a gas-lift. A mixture of liquid and gas (the latterpartly dissolved in the former) rises vertically from the oil-bearing layerthrough a cylindrical casing to the surface. In time conditions alter and thewell ceases to gush regularly, then the gushing can be further promoted byinserting a narrower tube in the well and connecting the top of the oil stringto the tubing. If the action in time becomes irregular, the gushing can be keptup for a further period by forcing gas between the two tubes. In the oil fieldsthe term" gas-lift" is used actually only where extraneous gas isapplied, as in the last of the stages mentioned. The action, however, is justthe same whether the gas exclusively originates from the formation, or ispartly applied artificially. Thus by gas-lift we simply mean a vertical tube inwhich the energy of gas under pressure, and of dissolved gas, is utilized forraising a liquid.
In gushing oil wells the pressure is frequently very high and the absorptioncoefficient 0.4 (expressed in vol. ratio) of the coexisting gas is notparticularly high, so that in reality it should be assumed that a considerableportion of the gas, at any rate at the bottom of the gas-lift, is dissolved inthe oil. For water-producing wells this is not usually of such importance.
High explosives, particularly nitroglycerin, have been used in torpedoes forthe purpose of shooting oil and gas wells for more than 60 years. The earlyhistory of the oil industry in Pennsylvania is not clear as to who actuallytorpedoed the first well, although in 1865 the Roberts Torpedo Co. procured apatent covering the process. Gunpowder was first used, although nitroglycerinwas substituted shortly afterwards.
Wells are shot for the purpose of increasing the flow of oil and gas. Ashot-hole in the producing horizon, with its contributory fissures andfractures, increases the area of and stimulates drainage into the hole. Theshot-hole also acts as a collecting basin from which the oil is pumped. As arule, hard or close-grained sands or limes are shot, other more or less porousand soft formations usually do not require shooting, and might be injured byblasting. Shooting is also resorted to in mechanical trouble such asstraightening crooked holes, sidetracking pipe or tools, and for severingfrozen strings of casing or drill pipe. Explosives are also used sometimes toextinguish oil or gas-well fires although that work, which involves unusualconditions and methods, does not properly come within the classification ofoil-well shooting.
Some Factors To Be Considered In Shooting Wells
Although nitroglycerin has been used extensively for more than half a centuryin shooting oil and gas wells, there is still a great deal of uncertainty as tothe proper method of shooting or the amount of explosive required to producebest results in a particular formation. The possibility of shootingunproductive or cavey formations above or below the productive horizons,shooting into lower water, destroying casing seats, and the splitting orcollapsing of casing strings, are factors that require consideration.
By unit operation is meant the developing and operating of oil pools underone management so as to make possible the application of the best principles ofbusiness, engineering, and science. The proposal is to do away with thedivision of ownership of the oil underground based upon property lines at thesurface and to recognize the fact that a pool of oil is a natural andindivisable unit that should be operated as such for the common good. But asunit operation upsets the customs of the industry and affects public policy, itis encumbent upon those of us who advocate it to show beyond reasonable doubtthat unit operation will result in worthwhile gains to both the property ownersand to the public. The purpose of the writers is to set out briefly theirconceptions of the benefits to be derived from unit operations in new poolswithout reference to any of the proposed means of consolidating the properties.Use is made of both their own experiences and of the proposals of the manyothers who have written or spoken on this subject.
Natural conditions vary widely in the different oil fields, and the exactmethods of unit development and operation must be varied accordingly. Also theeconomic value of unit operation will vary, being exceedingly large under someconditions and much less under others. It is not possible nor desirable in thispaper to cover more than the commoner conditions and to illustrate by someexamples, nor is this paper to be considered more than a preliminarypresentation on the subject for present methods of the industry are based uponcompetitive recovery, and it therefore is necessary for the industry to developby research and experience new concepts and methods for operating undernon-competitive conditions.
The most common type of oil pool is the one sand pool. The size may be froma few acres to several square miles. The oil is saturated with gas dissolvedunder pressure and the oil area is surrounded by water. When the pressure hasbeen taken off the pool, the production declines and leaves the largerproportion of the oil to be recovered by artificial means. Sometimes thesurrounding water encroaches and flushes much of the oil out of the sand, butmost commonly such encroachment is negligible. Less commonly there will be anexcess of gas not held in solution, which will be found as an area of free gasoverlying the oil or occupying the higher parts of the structure.
Gas-oil ratios in the production of oil have recently attracted theattention of production engineers throughout the country and much work has beendone in an effort to reduce the volume of gas produced with each barrel of oil,thereby increasing the ultimate oil production. When occasion arises to use therelationship of rate of oil production and rate of casinghead gas production ithas ordinarily been assumed to be within reasonably narrow limits constantthroughout the life of the property after flush production is past. Inoperating a lease for conservation of gas or more efficient methods of recoverythe rate at which the volume of casinghead gas is declining is as essential asthe rate of decline of oil production. Much work has been done on the rate ofdecline of oil production and while our knowledge of the subject is far fromcomplete it is much greater than our knowledge of the rate of decline of thecasinghead gas. The data presented show that a definite relationship existsbetween the rate of decline of oil production and volume of casinghead gas, andthat a study of this relationship will usually indicate the effect on thegas-oil ratio which may be expected when controlled pressure is applied.
Comparison of two curves with different rates of decline is difficult unlessone is expressed in terms of the other. Therefore in the discussion whichfollows the relationship of the rate of decline of casinghead gas to oilproduction is expressed by the cubic feet of gas produced with each barrel ofoil, or gas-oil ratio.
The subject of this paper might more appropriately be "The oil-gas ratioas related to the decline of casinghead gas." Where gas is the principalexpelling force of the oil from the sand, the oil is produced with thecasinghead gas rather than the casinghead gas being produced with the oil.Since oil production figures are more familiar and it is easier to follow fromthe known to the unknown, the more common usage is continued by expressing thecasinghead gas in terms of oil production, but it must be kept in mind that thereverse is the more exact condition.
When a flowing oil well is being drilled in with cable tools, it isdifficult to determine when an additional streak of pay sand is drilled. Byplacing a gas meter on the gas-release line from the oil and gas separator, anyincrease in volume of gas that comes with oil production is recorded. Inaddition to the better knowledge of sand conditions, these data are a valuableaid in determining the size and depth of shots, plugging off water, andestimating ultimate production. The meter also gives a record of operationwhile the well is drilling in; and if allowed to remain on the well aftercompletion will give information that may lead to a more efficient recovery ofthe oil from the sand.
A vertical cross section of any oil sand will show great variation inporosity and oil content. In some sands, the entire thickness contains oil, thevariation being only the degree of porosity or oil content, but more often thepay streaks are distinctly separated by shells or barren streaks. These paystrata vary in number and may be thin or thick. The total thickness of the sandmay be several tens of feet while the part containing oil may be only 1 or 2ft. thick. The amount of production is not proportional to the thickness of thesand but rather the thickness of the pay streaks.
If the total thickness of the sand or the amount that can be drilled issmall, or if the initial production is small, the depth at which each stratumof pay is encountered can be determined sufficiently accurately for all presenteconomic purposes. However, when drilling through sand of considerablethickness, giving large initial production, especially where conditionsnecessitate that the production go through the same gas trap with that from oneor more wells already producing, it is difficult to determine the depth atwhich the pay strata are encountered even accurately enough to guide theplacing of shots, and small pay streaks are apt not to be recognized atall.