Horner' and van Everdingen' have shown that the pressure drop within the wellbore, as a result of having produced the well at a constant rate q for time {, where t is sufficiently large, is:.6.pe = 47rk e h In /Lefr:van Everdingen observed that better agreement between theory and well performance can be obtained if, instead of assuming the permeability is ke everywhere about the well, it is assumed the permeability near the wellbore is substantially reduced as a result of drilling, completion andlor production practices. In order to account for the additional pressure drop he introduced the dimensionless quantity S, the skin effect factor, so that Eq. 1 becomes:.6.p, = 4::eh [ In ( /L~;~: ) + .809 + 2 s] .(2)Eq. 2 might have also been obtained as follows. Assume a zone of altered permeability k. exists about the well out to a radius r,,, and beyond that the unaltered, external permeability k e • The additional pressure drop required to overcome this skin of reduced permeability may be calculated with sufficient accuracy using the incompressible flow equation; for Brownscombe and Collins' have shown almost no difference between compressible and incompressible steady-state flow, in the vicinity of the wellbore, and the small volume of fluid in the vicinity of the wellbore makes unsteady-state mechanics unnecessary. Then, q/Lln(r.lrw) q0
A study of the North Ossun field, Louisiana, reveals that as reservoir pressure is depleted the increase in net overburden pressure initially pressure is depleted the increase in net overburden pressure initially causes rock failure and as the failure continues with decreasing pore pressure, rock compressibility decreases until eventually it reaches a pressure, rock compressibility decreases until eventually it reaches a normal value. Introduction Rock compressibility has long been recognized as an important factor in material balance calculations of oil in place for closed reservoirs producing above bubble-point pressure. For example, if the pore volume compressibility of the reservoir rock is half of the compressibility of the undersaturated oil, neglect of the rock compressibility term results in about a 50 percent overestimation of oil in place. In general, it percent overestimation of oil in place. In general, it may be stated that in material balance calculations on closed reservoirs, consideration of rock compressibility becomes increasingly important as the fluid compressibility decreases. For this reason the effect of rock compressibility is commonly neglected in studies on gas reservoirs where gas compressibility is usually great. Because gas compressibilities decrease with increasing pressures, the consideration of rock compressibility becomes increasingly important for deeper, high pressure gas reservoirs. For example, the compressibility of the gas in the reservoir to be discussed is 30 microsip** at an initial reservoir pressure of 8,921 psia. For a nominal pore volume rock compressibility of 6 microsip, neglect of rock compressibility in material balance calculations on a closed reservoir will result in a 20 percent overestimation of initial gas in place. If the rock compressibility is larger than 6 microsip, then a still larger over-estimation of gas in place results. In this study we propose that because of low net overburden pressures, rock compressibilities in geopressured reservoirs are considerably greater than for similar rocks in normally pressured reservoirs. We further suggest that as pressured reservoirs. We further suggest that as reservoir pressure is depleted, the increase in net overburden pressure initially causes inelastic rock compaction or rock failure. As failure continues with decreasing pore pressure, rock compressibility decreases and eventually reaches normal values in the range of 6 microsip. North Ossun Field, Louisiana The mechanisms proposed in the previous paragraph are believed to be illustrated by the performance of the NS2B reservoir of the North Ossun field, Lafayette Parish, La. This is a geopressured gas reservoir with an initial pore pressure of 8,921 psia at 12,500 ft subsea depth, or a gradient of 0.725 psi/ft. Table 1 gives pertinent data on this reservoir. Good geologic control is indicated by the structure map, Fig. 1. Although a gas-water contact exists, it is doubtful that the associated aquifer is very large because the reservoir appears to shale out on the west. In addition, considerable complex faulting in the area almost certainly closes the reservoir with a small associated aquifer. Good core and log data have been used to calculate an initial hydrocarbon pore volume of 583 million cu ft, and, with PVT data, to calculate an initial gas in place of 114 Bscf. JPT P. 1528
In recent years considerable attention has been given to the lise of unsteady-state pressure measurements in wells as a means of investigating the nature of petroleum reservoirs both near to and away from wells. These methods are based on sound theory; jor, theoretically at least, pressure build-up and drawdown curves reflect the nature oj the reservoirs jrom which the data are taken.One of the more common and prominent features oj reservoirs which can be detected in some instances by pressure measurements is the linear fluid-barrier, e.g., a linear sealing jault. There has been some misuse oj this technique in engineering practice, and it was therejore thought worthwhile to discuss the criteria jor valid pressure tests which indicate the presence and location of linear fluid-barriers.
Introduction Methods have been developed for drilling (a) slant holes and (b) one or more curved holes from a common central hole in producing formations. These wells will have productivities exceeding that of a single hole drilled normal to and fully penetrating the producing stratum. other factors being equal. This increase is due to the decrease in resistance to flow in the vicinity of the wellbore by an increase in the cross section exposed to flow with increasing footage drilled in the formation and due to the geometrical arrangement of the holes with respect to the drainage radius or boundary. Where other factors are not equal, for example, where zonal damage exists in the single, fully penetrating hole but not in the slant or curved holes, additional increases in productivity will accrue. Many of these wells have been drilled in the past and are currently being drilled with various results reported. While the authors are aware of some of the practical aspects of drilling and completing these multiple. curved holes, it is their hope to provide some basic data on the improvement in productivity to be anticipated in these wells for a number of hole arrangements or patterns. Model Studies Electric analogue or model studies have been used for solving some reservoir fluid flow problems in which the mathematical solutions are unknown, too approximate or too complex. For example, recent studies have used this method to determine the effect of shot density, diameter, and depth of penetration of gun perforations on well productivity. The success of these studies depends upon the analogy between Ohm's Law for electrical flow and Darcy's Law for incompressible fluid flow in homogeneous rock. Where a geometrical scale reduction is desired, a single scale factor is applied to all dimensions.
4 --G. 'more welle penetrete tha deeper hori--ABSTRACT One problem confronting! reeervoir engiaeers tcday la an analyeie of the L.-.--. reeervoir mocnanammm iii~~l?=d iil :h= depletion of abnormally hish preeeure reeervolre.Two mechanleme that have baen propoeed ara (1) hitth rock compreeaibility and (2) shale water influx. This paper la a study of the qhale water i"nflux theory. A litqrature search waa conducted to qetablieh what ie known qbout shala . ...--. .L. ,.--permeaox~azy, c0xIpres8aoLAALY, poroeity, and water viacoeity.The qhale properties were used in a calculation of water influx for two qctual auperpresaure reaervoira ualns a linear diffusivity equation in which permeability and compressibility were a function of praaeure. of the shale parametare , permeability qnd compreeeibility have the moat influence on water influx. For an initial shale permeability of tha ordar of 10-5 md and an Initial bulk compreeeibllity of 40 x 10-' pal-i , ehale watar infl~m eiSnificantly affacts the reservoir depletion.For shale permeabilftiea of tha order of 10-7 ad, ehale water influx ie insignificant.1: wae alao noted tnat the preaaure dietrlbution in the ah~le ia very steep and only the first few feet of shale contribute materially to the water influx. An approximate method for extrapolating qarly plz behavior in 9uPerPreaeure Sae reservoirs ia aleo preeented.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.