SPE AN ALTERNATIVE MODEL FOR FILTER CAKE FORMATION IN FRACTURES Randy Seright New Mexico Petroleum Recovery Research Center

TOPICS COVERED IN SPE New model for filter cake formation in fractures. 2.Effects of temperature and gel composition on extrusion for Cr(III)-acetate-HPAM gels. 3.Extrusion behavior for a guar-borate gel. 4.Effects of gel composition, fracture width, and addition of particulates on washout of gels from fractures (during brine injection).

WHAT ARE GEL PROPERTIES DURING EXTRUSION THROUGH FRACTURES? GEL USED: Cr(III)-acetate-HPAM  0.5% Ciba Alcoflood 935 HPAM, % Cr(III)-acetate, 1% NaCl, 0.1% CaCl 2, pH=6.  Gelation time ~ 5 hrs at 41°C.  Aged 24 hours at 41°C before use.  Injected into brine-saturated fractured cores, 41°C.

PROPERTIES OF Cr(III)-ACETATE-HPAM GEL DURING EXTRUSION THROUGH FRACTURES A minimum pressure gradient must be met before a formed gel will extrude through a fracture. Once the minimum pressure gradient is met, the pressure gradient during gel extrusion is not sensitive to injection rate. Once the fracture is filled with gel, pressure gradients are stable (no progressive plugging). The pressure gradient for gel extrusion varies inversely with the square of fracture width.

PROPERTIES OF Cr(III)-ACETATE-HPAM GEL DURING EXTRUSION THROUGH FRACTURES Gels dehydrate, thus retarding the rate of movement of the gel front. Although water leaks off through the fracture faces, crosslinked polymer cannot. Dehydrated (concentrated) gel is immobile. Mobile gel is the same as the injected gel. Mobile gel wormholes through immobile gel.

u l = 0.05 t -0.5 Average Injection Flux Leakoff rate = total matrix production rate total fracture area w f = 0.04 in.

WHY DOES u l = 0.05 t -0.5 ? Analogy with leakoff from hydraulic fracturing: u l = C 1 t -0.5 from SPE Monograph Volume 2 (1970), p. 33. Basis: Assume formation of a filter cake of uniform thickness, L, and permeability, k, with pressure drop,  p. u l =  p k / (  L) and L = C 2  u l dt Solution: u l = C 1 t -0.5 C 1 = [  pk/(  C 2 )] 0.5 porous rock fracture face filter cake, kL fluid flow in fracture water leakoff pp

WHY DOES u l = 0.05 t -0.5 ? Our problem is slightly different: The filter cake is not areally uniform. Fresh gel wormholes through concentrated gel.

Fraction of fracture area covered by immobile concentrated gel 1 minute 30 minutes Fraction of fracture area covered by mobile wormhole gel BASIS OF NEW MODEL (Model details are in SPE 75158): On first contact, fracture area is covered with mobile gel. With time, more and more concentrated gel accumulates. Area contacted by mobile gel decreases with time. Mobile gel forms wormholes through immobile gel. Dominant leakoff is from mobile (wormhole) gel.

CONVENTIONAL FILTRATION MODEL VERSUS THE NEW MODEL (SPE 75158) CONVENTIONAL MODEL ASSUMES: Areally uniform coverage with filter cake. Leakoff rate decreases with time because the filter cake becomes thicker with time. NEW MODEL ASSUMES: Two types of areal coverage Mobile gel—the dominant water source. Immobile gel—minor water source. Leakoff rate decreases with time because the area covered by mobile gel decreases with time.

u l = 0.05 t -0.5 w f, inches Gel: 0.5% HPAM, % Cr(III)-acetate, 41°C. Flux: 129 to 66,200 ft/d, L f : 0.5 to 32 ft, h f : 1.5 to 12 inches, w f : 0.02 to 0.16 inches. Summary of all tests to date at 41°C. New model u l = u m / [1 +  u l dt /w f ]

Conventional filter cake model New model Vertical views intersecting the fractures Porous rock Filter cake or concentrated gel Active flow path Fracture height { { Fracture width

Pressure gradients required to extrude a Cr(III)-acetate-HPAM gel through fractures dp/dl=0.02/w f 2

Conventional filter cake model New model Vertical views intersecting the fractures Porous rock Filter cake or concentrated gel Active flow path Fracture height { { Fracture width

Pressure Behavior in the Fracture During Gel Injection Gel injection rate = 200 cm 3 /hr or 413 ft/d, L f x h f x w f = 48x1.5x0.04-in. After 1 st gel contact, the conventional model predicts that flow channel width should decrease and pressure gradient should increase with gel throughput. The new model predicts that they should remain about the same.

CONCLUSIONS For extrusion of Cr(III)-acetate-HPAM gels through fractures in water shutoff applications, a new filter cake model was developed that: 1.is based on fracture area in contact with immobile concentrated gel increasing with time rather than filter cake thickness increasing with time. 2.predicts leakoff rates similar to those for the conventional filter cake model. 3.correctly predicts that pressure gradients during gel extrusion do not increase with throughput. 4.may be of interest in other areas (i.e., hydraulic fracturing, produced water re-injection).

New model for Cr(III)-acetate-HPAM 0.36% guar, 0.018% NaBO 2, 0.24% tallow soap, 0.1% surfactant, aged 1 day at 40°C, L f x h f x w f = 6x1.5x0.04 in., Injection flux = 4,130 ft/d. LEAKOFF RATES: Guar-borate vs. Cr(III)-acetate-HPAM Average dp/dl = 21.7 psi/ft, Breakthrough at 9.2 fracture volumes

0.36% guar, 0.018% NaBO 2, 0.24% tallow soap, 0.1% surfactant, aged 1 day at 40°C, L f x h f x w f = 12x12x0.04 in. Flow direction

Compared to the conventional filter cake model, if dp/dl and the total filter cake volume are fixed, the new leakoff model predicts: similar leakoff rate behavior, higher average fluid velocities, higher shear rates at the fracture wall, higher shear stresses at the fracture wall.

w c = flow opening for conventional filter cake model w w = wormhole width w w / w c > 1-1  n  0  w u ave ww dp/dl Constant pressure gradient (w w /w c ) 1/(n+1) (w w /w c ) (n+2)/(n+1) ww /wcww /wc 1 Constant injection rate wc /wwwc /ww 1(w c /w w ) (n+1) (w c /w w ) (n+2) PROPERTY COMPARISONS FOR A POWER-LAW FLUID: New Model Prediction Divided by Conventional Model Prediction