Gas content evaluation in unconventional reservoir

Transcription

1 Gas content evaluation in unconventional reservoir Priyank Srivastava Unconventional reservoirs 1

2 Average monthly prod. (mscf) The Problem Gas in-place calculation Prediction of production decline Total gas storage capacities and the resulting gas in-place values are being inadvertently inflated and overestimated. The source of error involves proper accounting of the volume occupies by the sorbed gas phase (Ambrose et al., 2010) Source : (lee & valko, 2010) Stabilized production, excess gas capacity?? Gas production from shales is the outcome of several different types of gas release processes, including, but not limited to, desorption 2

3 Outline Where are hydrocarbons stored Measurement of adsorbed isotherms Isotherm models & types Uses of isotherms Laboratory measurement of adsorbed gas in shale Discussions of results Methane adsorption in coal Workflow for gas in place calculations Intuition/ Resolution Mathematical Demonstration Experiments 3

4 We already have the KEY, All we have to do. is to find the LOCK Where are the Hydrocarbons? Gas in-place Total Gas Content Function of Pore Structure/Surface Area/TOC/Composition Free Gas Adsorbed Gas Stored by solution in matrix bituminite Stored in macropores Mono/Multi layer Micropore filling Capillary condensation Amt. adsorbed = f ( p, t, gas, solid) No sorbed gas?? Source : PE 3813, Well logs course slides,

5 Measurement of Adsorption Isotherms Direct Field Measurements Laboratory Measurements Seidle, John. (2011) Factors affecting the Field measurements of Gas content 1. Time taken for core retrieval 2. Lost gas/depth of core 3. Loss of Moisture and ash content 4. Full desorption may require months. and residual gas may not be measured Volumetric Method, Gravimetric Method, Gas chromatography Calibrate: Vo = no*vg, Vg = molar vol. Introduce Adsorbent: Vo V ; Wo w apparent adsorption = no V/Vg (Sing, 1982) The canister method is considered accurate to 30%. However, data apply only to a particular coal at a particular depth. More general estimate can be made by use of adsorption-isotherm data. Seidle, John. (2011) Calibrate & Measure Void volume Place Dry/Moisture free sample in adsorption cell Measure desorbed gas volume at different pressure steps Limitations : 1. Measurement usually done on dry samples. Even if cores are returned to as received moisture, adsorbed gas is not restored to original conditions 5

6 Isotherm Models & Assumptions Isotherm models Semi empirical equations Using standard isotherms shapes Langmuir Dubinin- Radushkevich Frankel-Hasley- Hill T-method, α-method Comparison Plots Solid surfaces are uniform: Langmuir, BET, Frenkel-Hasley isotherms Assumptions for site energy distribution: Freundlich and D-R equations Only dispersion forces are dominant in physical adsorption: Valid only when pore size is larger than molecular dimensions. Solid surfaces are not altered by adsorption: Fails in case of chemisorption/ matrix shrinkage Multilayer adsorbed films are bulk liquid like: in a thin multilayer region adsorbed films are perturbed structurally relative to bulk liquid Adsorption is negligible if the liquid adsorbate is non-wetting 6

7 Types of isotherm Type I (Langmuir isotherm) : Displayed by microporous solids with small external surfaces. Steep part represent micro pore filling. Low slope at plateau is due to multilayer adsorption being confined to a small external area. Estimation of surface area is doubtful. Type II : Exhibited by non-porous macroporous adsorbent. Point B is the stage where monolayer coverage is complete Type III : When adsorbent-adsorbate interaction is weak as compared to adsorbate-adsorbate interactions. (related to heat of adsorption) Type IV : Given by mesoporous adsorbents. Associated with capillary condensation in mesopores. Type V : related to Type-III but obtained with porous adsorbents. Type VI : Represents stepwise multilayer adsorption on a uniform non porous surface. Higher value of C (BET-parameter) in Type-III & Type-IV indicates Microporosity. Source : Sing (1982) 7

8 Isotherm Hysteresis and Pore shapes Type H1 : uniform spherical agglomerates Type H2 : pore shape not well defined. But occurs in pores with high aspect ratio(ink-bottle pores) Type H3 : plate like particles Type H4 : narrow slit like pores Low pressure hysteresis For H3 & H4 loops desorption curve will not yield correct surface area Source : Sing (1982) 8

9 Deriving Surface Area from isotherms Langmuir or BET isotherm can be used for external surface area estimation only if slope of isotherm is not too low at higher P/P o (since mesopores and external surface will be small in this case) and capillary condensation is absent. Derivative of isotherm can be used to visualize change of slopes at higher relative pressures. Fitting of some models over certain range of isotherm does not provide sufficient evidence for a particular mechanism of adsorption Microporosity can be assessed by pre-adsorption method in which micropores are filled with large molecules for isolating the micropores and leaving external surface for adsorption BET is unlikely to yield actual surface area if isotherm is of either Type-I or Type-III Gas used for measurement of isotherms may change the calculated results. 9

10 Effect of type of adsorbate on surface area calculations 131 units 396 Units Source : Sing & Gregg (1982) 10

11 Relating Isotherms to PSD Assumes Pores to be rigid & regular shape (cylindrical or slits), size distribution doesn't extend continuously from mesopore to macropore. Location and shape of desorption curve is dependent on which branch of the hysteresis loop is used to compute pore size. If capillary condensation occurs,psd from desorption curve is not valid. Mechanism of pore filling of micropores still unknown. The decision of which branch of hysteresis loop to use remains arbitrary. 11

12 Laboratory measurements of adsorbed gas (Volumetric method) Calibrate volume of reference & sample cell in presence and absence of nickel cylinder Place dry rock or kerogen sample in a adsorption cell And perform System leak test till pressure leakage less then 0.01 psia/hr. Measure Void volume at different pressure steps using inert helium gas. V void V r = ρ 2 ρ 3 ρ 3 ρ 1 Measure Adsorbed CH 4 on Kerogen or shale samples at various pressure steps. (Kerogen concentrate prepared using conventional demineralization) Limitations : 1. Effect of confining pressure on sample not included. 2. Gas densities calculated based on EOS models which may not be accurate 3. Averaging of different isotherms to obtain the type curve 4. Accounting Changes in permeability due to matrix shrinkage as gas desorbs. Source : Zhang et al., 2012 What can be done differently : 1. Measure Desorbed gas volume at decreasing pressures. 2. Applying confining pressure in adsorption cell. 3. Develop a approach to measure sorption & permeability simultaneously. 12

13 Langmuir Isotherm Interestingly, to date, virtually all experimental sorption data obtained from coals of all ranks and all geologic ages charged with pure gases of interest to coal gas engineering can be described with Langmuir s equation Seidle, John. (2011) p v = p v l + p l v l P l = The pressure at which the sample holds ½ the maximum gas volume (larger values indicate stronger affinity of the gas for the sorbent) V l = The Maximum amount of gas that can be sorbed onto given sample Vl needs to be corrected for ash and moisture content Relative efficiency of gas storage (EGS) : calculating equivalent porosity required to hold the same vol. of gas as sorbed in a sample. 1 P i eq S gi = V B l ρ B gi P i + P l 13

14 Langmuir isotherm Normalized with TOC Kerogens Type I (Green river) Type II (Woodford shale) Vl, Pl increases The aromatic kerogens (lower H/C ratio : Type III) have stronger affinity for methane than kerogens containing more aliphatic organic matter. Type III (Cameo Coal) Core Samples Green river Woodford Low maturity Barnett lee Shales Tarrant Pl, Vl increases Langmuir constant are greater for high maturity sample then for low maturity sample High maturity Blakely 14

15 Conclusions by Zhang et al. The lower level of gas adsorption in whole rocks indicate presence of inorganic minerals shields active sorption sites on the organic matter. For low thermal maturity samples, minerology doesn't play a significant role in gas adsorption within organic rich shales (this may not be true for other shale samples) Zhang s model developed for adsorbed gas on dry cores shows good correlation with moisture equilibrated conditions. They reasoned out this to be due to hydrophilic nature of clay mineral however, this needs further investigation. 15

16 Methane Adsorption in coals The amount of gas that a coal can adsorb varies directly with pressure and inversely with temperature V = K o P n o bt K o & n o are constants depending on rank of coal i.e. ratio of fixed carbon(fc) to volatile matter (VM) Thus, amount of gas in place adsorbed in coals can be written as : V = 100 %moisture %ash 100 V w [K V o P n o b 1.8h d ] V w /V d = ratio of adsorbed gas in wet and dry coal, Generally 0.75 K o, n o determined from dry coal adsorption isotherms Can we apply this methodology in shales? 16

17 Workflow to quantify gas in place Quantify Free Gas Measure Desorption isotherm Determine isotherm parameters for applicable desorption model Quantify Adsorbed gas volume V_fg (scf/ton) = 32.03*φ*(1-S w )/(ρbulk * Bg) V_adsorbed =K*P n b*t Factors affecting adsorption 1.) Surface area per unit mass of adsorbent 2.) Heat of adsorption G = H T S 3.) Physiosorption Vs Chemisorption 4.) Colloids Development of Averaging function to apply isotherm models on field scale. or Calculate Average Relative efficiency of gas storage. 17

18 The Future Downhole estimation of Langmuir gas sorption parameters (Gas content-pressure relationship) by use of well logging technologies Averaging function of isotherms based on organic matter characteristics to obtain a single isotherm applicable on reservoir scale. Isotherm derivatives. Simultaneous measurements of desorption isotherm and permeability changes Points to ponder Feasibility of gas desorption under operating Bottomhole reservoir conditions. What if : Gas in micro pores in actually in colloidal state : with dispersed phase as gas/liquid and dispersion medium as solids or liquids Effect of Hydraulic fracturing/changes in stress on adsorption equilibria 18

19 Thank You 19

20 Shortcomings of BET Theory Parameter C : Related to heat of adsorption only Qualitatively not Quantitatively. At low C it is not possible to determine n ma (point B) accurately. No mathematical description of Point B as of yet. So theoretical significance of amount adsorbed at B is questionable. Linearity of equation restricts the range of applicability. Knowledge of average molecular cross section area occupied by adsorbate molecule required. BET does not acknowledge micropore filling or cavities of molecular size Source : Sing (1982) 20

21 Applying Workflow to Zhang et al. data Formation ko no moisture% ash% Vw/Vd depth(ft.) b press. (psia) GIP (tcf) lee tarrant blakely ρ 2 = Gas density in the volume (V r +V void 1 ρ 3 = Gas density in V P 2 ρ 1 = Gas density in (V r +V void pressure decrease from P 2 to P 3 due to gas expansion N ads. = N total N ref. N void 21

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23 Kerogen Types Van krevelen diagram Espitalie diagram Type 1 Seprophilic H:C >1.25 O:C<0.15 Type 2 Planktonic H:C <1.25 O:C Type 3 Humic H:C <1 O:C 0.03 to 0.3 Type 4 Residue H:C < 0.5 Source : geoscienceworld.org 23