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A Quantitative Comparison of Some Mechanisms Generating Overpressure in Sedimentary Basins

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toyoovoS l\vC i^lt-^°°'/o62. A Quantitative Comparison of some Mechanisms Generating Overpressure in Sedimentary Basins Institutt for energiteknikk Institute for Energy Technology A quantitative comparison of some mechanisms generating overpressure sedimentary basins Magnus Wangen Institute for Energy Technology Box 40 N-2007 Kjeller NORWAY e-mail: [email protected] tel: +47-6380-6259 fax: +47-6381-5553 Submitted October 30, 2000 Revised Mars 7, 2001 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Performing Organisation Document no.: Date Institutt for energiteknikk IFE/KR/E-2001/002 2001-05-18 Kjeller Project/Contract no. and name Client/Sponsor Organisation and reference: Title and subtitle A Quantitative Comparison of some Mechanisms Generating Overpressure in Sedimentary Basins Authors) l\/j. 1 \ Reviewed >0 Approved Magnus Wan^cnj (j Tom Pedersen Tor Bjdmstad Abstract Expulsion of fluids in low permeable rock generate overpressure. Several mechanisms are suggested for fluid expulsion and overpressure build-up, and some of them have here been studied and compared. These are mechanical compaction, aquathermal pressuring, dehydration of clays, hydrocarbon generation and cementation of the pore space. A single pressure equation for these fluid expulsion processes has been studied. In particular, the source term in this pressure equation is studied carefully, because the source term consists of separate terms representing each mechanism for pressure build-up. The amount of fluid expelled from each mechanism is obtained from these individual contributions to the source term. It is shown that the rate of change of porosity can be expressed in at least two different ways for dehydration reactions and oil generation. One way in which the reduction in the solid volume enhances the porosity, and another where the porosity remains constant. A gravity number is defined by the permeability and the Darcy flux of the expelled fluid. The gravity number is shown to be a useful indicator for overpressure build-up. The gravity number is also used to estimate the thickness a seal needs for overpressure to rise from almost hydrostatic conditions abovethe seal to almost lithostatic conditions below the seal. Simple expressions for the Darcy fluxes caused by mechanical compaction, dehydration of clays, hydrocarbon generation and cementation of the pore space are derived. It is shown how these expressions can be combined with the gravity number to obtain an upper bound for the permeability for overpressure buiol-up to take place. Mechanical compaction and cementation of the pore space are shown to be the strongest expulsion mechanisms. Although most mechanisms can generate overpressure alone given a sufficiently low permeability, it is concluded that cementation of pore space is the most likely mechanism for overpressure generation in deeply buried reservoirs. Key Words Pressure build-up, mechanical compaction, aquathermal pressure, dehydration, hydrocarbon generation, cementation ISSN ISBN Numbers of Pages 0333-2039 82-7017-322-3 57 Supplementary Data Abstract Expulsion of fluids in low permeable rock generate overpressure. Several mechanisms are suggested for fluid expulsion and overpressure build-up, and some of them have here been studied and compared. These are mechanical compaction, aquathermal pressuring, dehydration of clays, hydrocarbon gen­ eration, and cementation of the pore space. A single pressure equation for these fluid expulsion processes has been studied. In particular, the source term in this pressure equation is studied carefully, because the source term consists of separate terms representing each mechanism for pressure build-up. The amount of fluid expelled from each mechanism is obtained from these in­ dividual contributions to the source term. It is shown that the rate of change of porosity can be expressed in at least two different ways for dehydration reactions and oil generation. One way in which the reduction in the solid volume enhances the porosity, and another where the porosity remains con­ stant. A gravity number is defined by the permeability and the Darcy flux of the expelled fluid. The gravity number is shown to be a useful indicator for overpressure build-up. The gravity number is also used to estimated the thickness a seal needs for overpressure to rise from almost hydrostatic condi ­ tions above the seal to almost lithostatic conditions below the seal. Simple expressions for the Darcy fluxes caused by mechanical compaction, dehydra ­ tion of clays, hydrocarbon generation and cementation of the pore space are derived. It is shown how these expressions can be combined with the gravity number to obtain an upper bound for the permeability for overpressure build ­ up to take place. Mechanical compaction and cementation of the pore space are shown to be the strongest expulsion mechanisms. Although most mech ­ anisms can generate overpressure alone given a sufficiently low permeability, it is concluded that cementation of pore space is the most likely mechanism for overpressure generation in deeply buried reservoirs. Key words: pressure build-up, mechanical compaction, aquathermal pres ­ sure, dehydration, hydrocarbon generation, cementation 0 1 Introduction Overpressure is generated when fluids are expelled from low permeable rocks. Expulsion of fluids and the associated overpressure build-up have been suc­ cessfully modeled as mechanical compaction during deposition of sediments, (Lerche 1990, Ungerer et al. 1990, Audet 1996). Mechanical compaction is here understood to be compaction where the porosity is controlled by the effective vertical stress. However, several other mechanisms are proposed for overpressure build-up. Among these are thermal expansion of the pore fluid, dehydration of clays, hydrocarbon generation, and cementation of the pore space, (Osborn and Swarbrick, 1997). It is therefore a need for means to compare the different mechanisms. The study aims at this with simple ex­ pressions which make comparisons possible. This study is based on a pressure equation which account for the above mentioned mechanisms for overpressure build-up. This equation is derived from conservation laws for each species (or mineral) in a multi-component medium and a conservation law for the fluid. These conservation laws have source terms which accounts for the minerals and the fluid being produced or consumed. Modeling some of the pressure generation mechanisms therefore ends up in selecting proper source terms to represent the different processes involved. Another important issue in over ­ pressure modeling is how the rate of change of porosity is modeled. The choices made here imply that the equation for the overpressure becomes a parabolic differential equation. The different source terms for each pressure generating mechanism are seen to be added together as one effective source term for overpressure build-up. A study of each single source term allows the relative importance of the different mechanisms to be studied individually. The source terms in the equations for overpressure build-up have been identi ­ fied and studied earlier in the case of aquathermal pressuring. Both Shi and Wang (1986) and Luo and Vasseur (1992) discuss aquathermal pressuring based on source terms. This analysis considers a wider range of mechanisms than aquathermal pressuring. The expulsion rates of the different overpres ­ sure generating mechanisms are integrated to Darcy fluxes. These Darcy fluxes allow the relative contribution of the different mechanisms to the total Darcy velocity to be studied. These Darcy fluxes are combined with a gravity number which indicates when there will be high or low overpressures. The gravity number also allows for a simple estimate of how thick a seal ought to be for high overpressure to develop across the seal. 1 This study is based on a 1-D pressure equation with only one lithology, which is a common setting for a large number of models studying compaction and fluid flow processes in sedimentary basins, (Gibson 1958, Bredehoeft & Han- shaw 1968, Smith 1971, Sharp and Domenico 1976, Bethke & Corbet 1988, Autdet & Fowler 1992, Wangen 1992, Birchwood & Turcotte 1994 Schneider 1996, Fowler & Yang 1999, L’Heureux & Fowler 2000, Yang 2000). However, the pressure equation is derived in 3-D and then simplified to the verti­ cal direction. Particular concern is paid to the definition of excess pressure, boundary conditions and how Darcy’s law is expressed in the excess pressure. The current study does not consider all possible models for compaction of sed­ iments and the associated pressure build-up. In particular, viscious rehologi- cal models of the sediments are not considered, see for instance Birchwood & Turcotte (1994), Schneider et al. (1996), or Connolly & Podladchikov (1998, 2000), Suetnova & Vasseur (2000). Although such models lead to a different type of pressure equation than the one studied here, the same methods to compare mechanisms for overpressure generation could be applied. Other mechanisms which are not accounted for, but can generate overpres­ sures in the subsurface, are cracking of oil to gas and meteoric fluid flow. Oil to gas cracking expels fluids because of the associated volume expansion. Meteoric fluid flow can lead to high overpressure in formations with good lateral permeability. This paper is organized as follows: The basic assumptions like the conser­ vation laws are presented
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