3Formation DamageMathematically, formation damage is a reduction in the flowing phase mobility, l:13
4Formation DamageFor the oil phase, the hydrocarbon effective mobility, lo, is:13
5Asphaltene-Induced Formation Damage Mechanisms Absolute permeability impairment (k)Wettability Changes (kro)Viscosity (mo) increase due to:Emulsion formationAsphaltene particle increase near the borehole
6Asphaltene-Induced Formation Damage Mechanisms From the previous three mechanisms of asphaltene-induced formation damage the first one appears to be the dominant mechanism, although occasionally the second and third mechanisms do seem to play a role under certain circumstances. If there is no water production, which is the most likely case, then no emulsion of water-in-oil is expected. Hence, any viscosity increase measured in the laboratory would have to be attributed to asphaltene particle concentration increase as the reservoir fluid approaches the wellbore. Past experiments have shown that asphaltene flocculation in-of-itself does not result in a significant viscosity increase. Also, from experience, reservoirs that have asphaltene problems seem to be mixed-wet to oil-wet even before production commences. It evident then that the major cause of asphaltene-induced formation damage in asphaltenic reservoirs is the first mechanism.
7Physical Blockage of Pore Throats Caused by In-Situ Asphaltene Deposition 13
8Reservoir characterization Reservoir characterization is an enormous subject that consumes a lot of manpower energy in the oil industry. Some of the parameters that characterize a reservoir are:k, permeabilityf, porosityRQI, Reservoir Quality Index (mean hydraulic radius)Fluid SaturationsWettabilityElectrical Properties (Formation Factor and Resistivity Index)
9Formation Mean Hydraulic Radius Pore throat radii of a formation depend on many reservoir parameters and they certainly vary from one formation to another. However, some general statements about the distribution of hydraulic radii of formations can be made:Largest distribution is to 100 micronUsual distribution is 0.01 to 10 micronOccasional distribution is 0.1 to 1 micron
10Flocculated Asphaltene Micelles Forming an Asphaltene Particle 12 Å45 Å
11Obvious question: How can so small flocculated asphaltene particles plug pore throats of much larger size?
12Physical Blockage of 100 m Pore Throat Caused by In-Situ Flocculation of Much Smaller Asphaltene Particles100 m
13Physical Blockage Caused by In-Situ Asphaltene Deposition-Sandstone 13
14Asphaltenes Adsorbed on the Rock Cause Wettability Changes 12
15Wettability Change Caused by In-Situ Asphaltene Deposition-Carbonate 13
16Asphaltene-Induced Formation Damage Near Production Wells High draw-downMiscible-gas breakthroughContact of oil with incompatible fluids during drilling, completion, stimulation, fracturing, and gravel packing operationsDrop in reservoir pressure below onset of asphaltene pressure
17Well Producing with Asphaltene-Induced Formation Damage 1st Mechanism2nd Mechanism
18Well Producing with Asphaltene-Induced Formation Damage
19Well Producing with Asphaltene-Induced Formation Damage The previous slide shows an aerial view of a producing well suffering from asphaltene-induced formation damage. The well is on flow control from the choke. As Pw drops, when asphaltene deposition starts, the choke opens so that the lower Pw can push all of the oil flow, q, through the tubing. Pe and PAF remain constant. However, when the choke is completely open both re and rAF begin to decrease due to the production rate decrease caused by the ever-increasing asphaltene-induced formation damage. When rAF becomes equal to rw no additional formation damage is incurred. This is referred to as "true steady state condition". The production rate at this state, however, may not be economical.
25Well Producing with Asphaltene-Induced Formation Damage 1st Mechanism2nd Mechanism
26Hydraulic Radius – Single Channel The hydraulic radius of a single flow channel is given by:Where:S = x-sectional area of flow channelLP = wetted perimeter of flow channelL = length of flow channel
27Hydraulic Radius – Core Plug The hydraulic radius of a core plug is given by:Where:f = core plug porosity (= void volume/total volume)SP = surface area of one core plug grain or particleVP = volume of one core plug grain or particle
28Hydraulic RadiusBy further mathematical manipulation, the hydraulic radius of a core plug is given by:Where:f = core plug porosity (= void volume/total volume)dg = average grain diameter. There are proprietary correlations for sandstones and carbonates that allow one to calculate dg from k and f.
29Mean Hydraulic Pore-Throat Radius A simple, quick and dirty way to estimate the Mean Hydraulic Pore-Throat Radius is via the following equation:
30Retained Asphaltene Particle Diameter, dAP The simplest rule-of-thump from filtration theory is that a filter retains particles with diameters 1/3 of the nominal rating of the filter. In this case, the filtration rule-of-thump means that:Where:dAP is the diameter of the average size asphaltene particle retained by the formation
31Physical Blockage of 100 m Pore Throat Caused by In-Situ Flocculation of Much Smaller Asphaltene Particles100 mm
32Retained Asphaltene Particle Diameter, dAP In the more general case, however, a more appropriate definition for dAP is:Where:a is a constant that accounts for the variation of the size of the asphaltene particle filtered by the formation. a varies from 0 to 1.dH is the average hydraulic diameter of the producing horizon
34Darcy's equation for steady state radial flow is: Where:m is viscosity, centipoiseq is reservoir barrels per dayk is permeability, DarcyP is pressure, psiar is distance from center of wellbore, feet
35The Darcy equation applies to each radial segment Dr at location r
36Initial Area Available to Flow Ainitial(r) At time equal to zero, i.e., before any asphaltene plugging, the total area available to flow at a distance r from the center of the wellbore is:Where:h is the net thickness of the formation or the net pay zonef is the initial average effective porosity of the formation
37Net Area Available to Flow Anet(r,t) The net area available to flow at location r, after asphaltene plugging for time t, is obtained as follows:Where:AAP(r,t) is the total area plugged by asphaltene particles at location r at time t. The calculation of AAP(r,t) is described next.
38Total Area Plugged AAP(r,t) The total area plugged by asphaltene particles at location r at time t is AAP(r,t):Where:DAAP(r,j) , is the incremental area plugged by asphaltene particles at location r within time interval jN is the number of time intervals
39Calculation of Incremental Area Plugged DAAP(r,j) Very Effective Pore-Throat Plugging by Least Number of Asphaltene ParticlesPore-ThroatAsphaltene Particle
40Calculation of Incremental Area Plugged DAAP(r,j) The incremental area plugged by asphaltene particles at location r within time interval j, DAAP(r,j), is:Where:IMAT, is the number of incremental moles of asphaltene particles being trapped at location r within time interval jMVA, is the molar volume of asphaltene particles at location r at time interval jCSAAP, is the cross-sectional area of the average size asphaltene particle retained by the formation at location r at time interval jVAP, is the volume of the average size asphaltene particle retained by the formation at location r at time interval j
41Calculation of Incremental Area Plugged DAAP(r,j) The equation giving the incremental area plugged by asphaltene particles at location r within time interval j, DAAP(r,j), is:Where:DAPtrap(r,t), is the number of incremental moles of asphaltene particles being trapped at location r within time interval juA(r,j) is the molar volume of asphaltene particles at location r at time interval jdAP, the diameter of the average size asphaltene particle retained by the formation
42Calculation of Incremental Area Plugged DAAP(r,j) After rearrangement and substitution, the equation giving the incremental area plugged by asphaltene particles at location r within time interval j, DAAP(r,j), is:Where:DAPtrap(r,t), is the number of incremental moles of asphaltene particles being trapped at location r within time interval juA(r,j) is the molar volume of asphaltene particles at location r at time interval jdAP, the diameter of the average size asphaltene particle retained by the formation
43Total Area Plugged AAP(r,t) Substitute into the equation giving the total area plugged by asphaltene particles at location r and time t, AAP(r,t), to get:
44Total Area Plugged AAP(r,t) Hence, to calculate the total area plugged by asphaltene particles at location r at time t, AAP(r,t), we need the following:dAP, the diameter of the average size asphaltene particle retained by the formationuA(r,j), the molar volume of average size asphaltene particles at location r at time increment jDAPtrap(r,j), is the number of incremental moles of asphaltene particles being trapped at location r within time interval j
45Average Diameter of Asphaltene Particles Retained Remember that dAP is the diameter of the average size asphaltene particle retained by the formation and is given by:
46Molar Volume of Asphaltene Particles Retained uA(r,j), the molar volume of asphaltene particles at location r at time interval j, is calculated by the phase behavior model. In this case, it is calculated by the TC Model, AsphWax’s asphaltene phase behavior simulator.
47Incremental Moles of Asphaltene Particles Retained DAPtrap(r,j), the incremental moles of asphaltene particles retained, is obtained as follows:At the pressure and temperature prevailing at location r at time t, the asphaltene phase behavior model (TCModel) calculates the moles of asphaltene particles per mole of reservoir fluid, s, and their psd, f(x), where x is the asphaltene particle diameter.f(x) is then integrated from x = dAP to x = ¥ to obtain the moles of asphaltene particles being trapped, ftrap, per mole of reservoir fluid at location r within time interval Dt.The total number of moles of reservoir fluid, MRF, flowing at location r is obtained by flowing the well at flowrate q for some specified production time interval Dt.8
48Incremental Moles of Asphaltene Particles Retained Hence, from a material balance, the number of incremental moles of asphaltene particles being trapped at location r within time increment j, DAPtrap(r,j), is:
49Total Area Plugged AAP(r,t) Substitute into the previous equation to get the total area plugged by asphaltene particles at location r at time t, AAP(r,t), as:Where:g is a constant whose value is greater or equal to 1. it is related to a by the relation g=1/a. Recall that a varies from 0 to 1. g (or a) indicates the "efficiency" of plugging of the asphaltene particles. It may be used as a tuning parameter, if well history-matching data are available.
50"Degree of Damage", DODIt is convenient to introduce the "Degree of Damage", DOD, at each location r at time t. DOD may be defined as:
51Permeability Impairment Using the definition of DOD and applying Darcy’s law to both the damaged and initial situation one gets:
52"Degree of Damage", DODAfter further substitution and simplification one gets:
53Permeability Impairment The previous equations yield the definition of damaged permeability:The damaged permeability, kdam(r,t), at location r and time t is used in the Darcy equation to calculate the pressure drop in the formation. Although this gives the appearance that permeability is treated as a point property, in reality it is not. k does not really refer to a point at distance r but rather to an increment Dr at distance r from the center of the wellbore.
55Porosity LossPorosity loss is modeled in a similar way to permeability. Using the definition of porosity we have:
56Dividing both sides of the above equations, one gets: Porosity LossDividing both sides of the above equations, one gets:If the radial interval Dr is taken very small we can assume that the area loss at this interval due to asphaltene flocculation is uniform across its thickness Dr. This implies that DOD(r,t), as previously defined, is an average value for the whole increment thickness Dr at location r.
61Asphaltene Deposit Erosion As plugging is occurring inside the asphaltene-inflicted formation, the interstitial velocity increases continuously at constant production rate (constant rate accomplished by opening the choke). If the system continues to produce undisturbed, the interstitial velocity eventually becomes equal to and surpasses the critical velocity at which point previously deposited asphaltene particles begin to move with the flow. At further velocity increases the rate of erosion becomes equal to the rate of deposition, hence, no additional asphaltene damage occurs at that location. Such a “pseudo-steady state condition” has been observed in the field in wells undergoing asphaltene deposition in the near-wellbore formation. The production rate at this steady state condition, however, may not be (and generally it is not) economically acceptable or viable. In general, when the steady state condition is reached or even before then, the well requires stimulation to restore production at economical rates. It should be noted that some of the smaller pore throats may plug up completely. Whereas the bigger pore throats may reach the steady state condition caused by the deposit erosion rate being equal to the deposition rate.
62Well Producing with Asphaltene-Induced Formation Damage 1st Mechanism2nd Mechanism
63Asphaltene Deposit Erosion At the previously described pseudo-steady state condition, the net area available to flow Anet(r,t) corresponds to some fraction of the initial area available to flow Ainitial(r). The following equation is recommended for achieving steady state condition in the simulation:
64Asphaltene Deposit Erosion There are certain salient features of the previous equation that make it suitable for representing asphaltene particle erosion at the steady state condition. First, it is evident that as the degree of damage, DOD, increases the value of DA(r,t) approaches B*Ainitial(r). This is equivalent to saying that:Hence, the value of constant B is limited to 0 < B < 1. B places a limit on the maximum damage that can occur at location r. For those Dr segments or locations that reach the maximum deposition at some time t, DOD will remain constant after time t. This will show as a decline in thedamage rate increase, such as a decline in the skin factor increase, when a segment reaches its maximum deposition level specified by B. Factor B can be used for history-matching data, along with constant g.
65Asphaltene Deposit Erosion It is evident that at time zero the degree of damage is equal to 1. That is DODt=0=1. Also, at time zero Anet(r,t=0) = Ainitial(r), because AAP(r,t) = 0. Hence, from the previous equations one can derive that:A + B = 1A consequence of the above equation is that the constants A and B are not independent. If one is specified the other is obtained from the above equation. Also, since 0 < B < 1, it follows that 0 < A < 1.
67Solution Algorithm Where: The key to solving the set of equations in this formation damage model lies in solving the equation shown below numerically. As already mentioned, the near well formation is broken into small equal segments Dr and the production time in small intervals Dt, i.e., this is a finite difference approach.Where:i, refers to radial segment Dr at position i in the formationj, refers to time intervals j and varies from 1 to N.N,is the total number of time intervals. Hence, total production time is equal to N*Dt.
68Solution AlgorithmThe near-wellbore region is divided into radial segments Dr. The formation damage calculation is performed for all segments at each time interval Dt.
69Solution AlgorithmPe and PAF remain constant while PW is declining. Flow, q, is constant. During time interval j, PW remains constant. Say for time interval 6, PW is P6, for time interval 4, PW is P4, etc.
70Solution AlgorithmThe previous figure shows the decline in the bottom hole pressure Pw with time caused by asphaltene-induced formation damage. An analogous decline is taking place in the entire region affected by asphaltene deposition (r£rAF). One can assume that during a time interval Dt the pressure is equal to an average value Pavg. This allows one to calculate the thermodynamic properties of the reservoir fluid and utilize the previous equation as if during time interval Dt the system was at steady state (although during time interval Dt some incremental formation damage is taking place). The accuracy of this approach obviously improves with smaller time interval Dt and radius segment Dr, because the smaller the time interval and radius segment the closer the system is to steady state. This is true with any other numerical finite difference approach.
71Solution Algorithm The calculation proceeds as follows: Using the well productivity index, PI, at time t=0 calculate a starting Pw that corresponds to the production rate q. Then, the formation hydraulic diameter dH and average retained asphaltene particle diameter dAP are calculated. Also, the mole rate of reservoir fluid, MRF, corresponding to q is calculated.Assuming no damage present, the pressure profile is calculated from the wellbore to the well drainage radius, re, using the Darcy equation (i.e., DPs=0). This gives us the undamaged pressure profile at time t=0.At the first time interval Dt, i=1, the FDModel starts the calculation at r=rAF, because if q remains constant rAF remains constant as well. Hence, the pressure profile at locations with r³rAF remains constant during the pseudo-steady state period (i.e., there is no damage at r³rAF). But the pressure profile changes continuously at r£rAF.
72Solution AlgorithmFor every radial segment inside the damaged area (r£rAF), the asphaltene model is used to calculate the psd f(x), the mole fraction S, and molar volume uA of the flocculated asphaltene particles.The asphaltene psd, f(x), is then integrated for x=dAP to infinity to obtain the mole fraction of asphaltene particles trapped, ftrap.The derived equation is used to calculate the area plugged by the asphaltene particles at location i and time interval j, AAP(r,t).DOD is calculated next.Then, kdam, fdam, DPdam, DPs, and s are calculated.The damaged permeability kdam obtained in the previous step is used in the Darcy equation to calculate a new pressure profile for the region r£rAF.The last 6 steps are repeated for the next time interval Dt, j+1.
73Solution AlgorithmThe calculation proceeds until one of the following happens:The time t=j*Dt reaches tmax, the total production time specified at the beginning of the calculation.the choke opens all the way and the flow rate cannot be kept constant. At this time, the pressure profile at r³rAF begins to change also due to the decreasing flowrate. This corresponds to some low Pw. This low Pw must be specified to the model as a model stopping criteria.