Sand retention testing for screen selection in soft-sand completions has become an integral component to verify the screen performance. This paper describes an improved constant flow-rate test method developed for sand retention testing. The improved constant flow rate test method simulates an erosional-type failure of the formation onto the gravel pack or screen. This test methodology allows the sand to be injected at known concentration into the brine stream, eliminating mixing and settling issues. The constant flow rate test procedure utilizes a concentrated slurry of the formation sand in brine. This slurry is then injected into the brine flow-stream to accomplish the desired solids concentration for the test (typically 0.5-1.0%). The method allows for flexibility both in the brine flow-rate used and in the solids concentration tested. The slurry injection is accomplished in a slot-flow cell and is designed such that the brine stream erodes the particles from the face of the slurry as it is injected. The distance to the screen after the solids injection is minimized to overcome faster settling of larger particles. The constant rate sand retention test method provides the amount of solids produced through the screen, the size of the solids produced through the screen, the retained permeability of the screen, and the pressure increase over time as the formation is deposited on the screen. Detailed in the paper is a reproducibility study that was included in order to verify the test method. Additionally, a comparison between the constant flow rate and constant drawdown test has been conducted using the new constant flow rate test method. The new constant flow rate test method has been found to have greater reproducibility than the previous method. Additionally, the constant flow rate and constant drawdown test have been found to produce similar test results with known exceptions that are detailed in the finding. The improved constant flow rate test method provides more consistent, reproducible results to assist in determining the optimal sand control screen for a target completion.
Fracturing fluids have traditionally been viscosified with guar and guar derivatives. Non-acetylated xanthan is a variant of xanthan gum which when combined with guar in solution develops a synergistic interaction that generates superior viscosity and particle transport at low polymer concentrations. These water-base linear fluids have improved low shear viscosity at concentrations at or below 25 lb/1,000gal when compared to fluids viscosified using a single viscosifier such as guar or xanthan gum. The polymer mixtures can be crosslinked to provide enhanced viscosity at higher temperatures. Introduction The main function of fracturing fluids is to open the fracture and transport proppant along the length and height of the fracture. The rheological properties of the fluid are considered basic for these functions. Water-based hydraulic fracturing fluids have been generally viscosified with guar and guar derivatives. The guar molecules are normally crosslinked with different crosslinkers.1 However, optimum hydraulic fracturing treatments require that the fluid cleans out of the fracture, leaving minimum damage to the proppant pack. The damage or reduction in permeability to the proppant pack is due to polymer residue leftover from the fluid. In order to reduce the damage leftover by the polymer, oxidizers and enzymes are used to break down the molecular weight of the polymer.1 However, the effect of the breakers is still not enough to eliminate the damage to the proppant pack. Usually the majority of the polymer remains in the fracture and does not flow back.2,3 As a consequence, polymer loadings were reduced from the 30–40 lb/1,000gal to 15–25 lb/1,000gal.4,5 Fluid formulations were adjusted to still yield acceptable rheology for proppant transport after crosslinking the polymer. No adjustments were made for the linear gel at the surface, where the viscosity of low-polymer linear gels is very low. Alternatives to polymers are surfactants, which form worm-like micelles in solution.6 These micelles have a gross structure similar to polymer chains, which increase the viscosity of the fluid. The complete breakup of the fluid occurs when the polymer enters in contact with hydrocarbons. However, emulsions have been known to occur with some crude and the cost of these fluids is high in comparison to polymer-based fluids. In this paper a new technology is presented which enhances the viscosity of linear gels using a synergy between guar and Non-Acetylated Xanthan (NAX), a variant of xanthan gum. The synergy develops when NAX is combined with guar in solution yielding higher viscosities than guar or xanthan alone at the same total polymer concentration. The effect of different parameters on the rheology of the mixtures is presented, such as the ratio of guar to non-acetylated xanthan, the effect of salts, temperature and shear history. Large scale proppant transport tests were performed to evaluate proppant transport of the mixtures with respect to pure guar fluids. Background Guar gum is a non-ionic galactomannan, which is extracted form the endosperm of guar beans. The monomer unit of guar gum is composed of linear chains of ß-D-mannopyranosyl units linked to each other by 1–4 bonds. Also a single a-D-galactopyranosyl unit is linked to the mannose by a 1–6 bond. The ratio of mannose to galactose varies from 1.6:1 to 1.8:1, as opposed to 2:1, indicating some unsubstituted backbone.1
A suite of practical laboratory tests has been developed to characterize the performance of shale fracturing fluids and chemicals. The testing suite includes friction reduction, capillary suction time, and shale strength changes after exposure to the fracturing fluid. The friction testing method was correlated to previously published work in larger diameter tubing and utilizes a small volume of water so that actual field water samples can be used. The capillary suction utilizes off-the-shelf equipment to give rapid determination of fluid and shale interactions. The shale strength change method involves dynamic exposure of shale cores to the fracturing fluid at temperature, shear rates and times representative of the fracturing treatment. Shale strength changes are determined by measuring hardness of the core surface exposed to the fracturing fluid and comparing it to the opposite side of the core which was not exposed to fluids. The friction reduction test has been used with great success to rate the performance of friction reducers in various types of water, to determine optimum friction reducer concentration, and to determine the interaction of other common additives such as biocides or scale inhibitors with the friction reducers. The strength testing has shown significant strength reduction in some shales with certain types of fracturing fluids and little strength reduction when the fracturing fluid was changed. A variety of fracturing fluids have been tested from slickwater to gelled oil fluids to traditional crosslinked fluid. The strength reduction should be indicative of proppant embedment in the shale. The suite of tests proposed has been successfully used for multiple operators in order to determine interaction effects of fracturing fluids with shales as well as to optimize friction reducer performance.
Traditional sand control sizing has typically been based on "standard", wide-sieve gravel distributions (i.e. 20/40, 16/30, etc). Historic sand retention testing has therefore been limited to these standard gravel (i.e. proppant) sizes. With the emergence of new proppant technologies, extensive testing has recently been performed to evaluate the impact of mono-sieved gravel on sand retention performance. Sand retention testing was performed using a number of industry test protocols [Martch 2012] to ascertain the impact of sieve distribution on gravel sizing rules. The testing involved multiple formation particle size distributions (PSDs) and compared the sand retention characteristics of standard-sieve gravel, to comparably sized mono-sieve gravel. Over a dozen PSDs were taken from actual formations containing both uniform and non-uniform distributions, over a wide range of mean particle diameters (d50). Multiple gravel sizes were also tested. Performance indicators measured include produced solids, size of largest produced solids and retained gravel permeability. Comparison of the mass of produced sand through various combinations of formation/gravel are useful in identifying the preferred gravel to manage solids production. This study will show that sand control performance of mono-sieved gravel is comparable to that of standard-sieve distribution gravel. This is illustrated by comparing the mass of produced sand and measurement of permeability in the various formation/gravel combinations. The paper will demonstrate that numerous "rules of thumb" employed for gravel sizing (including use of "Saucier's ratio") during the gravel- and frac-pack design process can be applied to any sieve distribution gravel, whether standard- or mono-sieved. In addition to the test results, this paper will reference multiple GOM applications with frac-pack completions in which sand control is performing as designed using mono-sieved gravel. This paper is critical for all completions engineers who are designing gravel or frac-pack completions. Sand retention testing on mono-sieved gravel is novel, and these results complement existing testing. The results of this testing have already been applied by several exploration and production companies, and this paper will allow others to benefit from the work.
One of the many benefits of installing open-hole completions in high angle or horizontal wells is higher well productivity compared to cased and perforated completions. This is particularly true in applications requiring down-hole sand retention. However, earlier studies have shown that productivity of open-hole completions can be impaired by drilling fluid filtercake or filtercake residue. Unfortunately, to obtain a complete gravel pack in an open-hole section, it is necessary to leave the filtercake intact. Once the gravel is placed, this filtercake becomes trapped between the borehole wall and the gravel pack. The final disposition of this filtercake layer has been the source of much speculation over the past 10 - 15 years. Many proponents of open-hole gravel packing contend the filtercake ultimately disperses into discrete components and passes through the gravel pack and screen without impairing productivity. Others suggest that the filtercake is more tenacious and is breached in random, limited locations along the length of the completion leaving the majority of the filtercake intact. If the filtercake is not effectively removed and inflow area is significantly reduced, areas of concentrated flow commonly referred to as "hot-spots" can be created leading to impaired productivity and/or completion failure. This paper presents a laboratory investigation of open-hole gravel pack impairment from drilling fluid filtercake conducted under simulated down-hole conditions. A limited number of commonly-used Drill-in Fluid (DIF) formulations were selected for this study along with unconsolidated, formation sand samples obtained from whole core. Gravel sizes used in the study were based on the median grain size of the formation sand and ranged from conservatively sized (D50/d50 ~ 2.0) to aggressively sized (D50/d50 > 8). Results of the tests show the extent of filtercake damage to both the formation sand and the gravel pack as well as the depth of permeability reduction. Finally, the benefit of placing a remedial treatment in the open-hole section following the gravel pack is discussed.
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