David L. Lord

Gravel Pack Designs of Highly-Deviated Wells with an Alternative Flow-Path Concept

Alpha-Beta gravel packing procedures have been used with a moderate degree of success in highly -deviated wells. Incorrect concentrations of gravel and/or pump rates can result in bridge formation in the open hole/screen annulus and Beta wave initiation prior to reaching the toe. If there is a high leakoff zone, gravel concentration will increase, and there may be insufficient velocity to trans port the solids farther down the well. Either factor or a combination of the two can lead to formation of a bridge in the openhole/screen annulus and an early initiation of the Beta wave. Other effects that could lead to bridge formation include: flow restriction and blockage from collapse of an unstable open hole section, and changes in annular velocity transition from one hole size to another. Incomplete gravel placement and the presence of voids around the screen can result from all of the complications described above. To overcome these, an alternative flow path system has been developed. If a bridge forms, the alternative path allows the slurry to bypass it. A number of physical models have been used to design and examine the effectiveness of the system, which has been validated in field applications. A numerical model has also been developed to assist with the gravel pack designs in highly-deviated wells. The model simulates the alternative flow -path concept as well as conventional gravel packing in open hole or cased -hole completions of arbitrary deviation. Details of the alt ernative flow -path scheme as well as the formulation of the numerical model are presented in this paper. Simulation results were compared to observations in the physical models.

Static/Dynamic Settling of Proppant in Non-Newtonian Hydraulic Fracturing Fluids

Experimental data and analysis are presented on the static and dynamic settling of proppant in non-Newtonian hydraulic fracturing fluids. A unique High Pressure Simulator (HPS) that closely models reservoir hydraulic fractures is used to study proppant-settling behavior under both static and dynamic conditions. A system of optical fibers and Light Emitting Diodes (LED) acting as a vision system were imbedded in the platens representing fracture walls for quantification of proppant settling under various fluid injection conditions. Hydraulic fracturing fluids tested include slurries of 35 lb/Mgal guar linear gel with 6 ppg 20/40 mesh sand, 60 and 40 lb/ Mgal hydroxypropyl guar (HPG) linear gels both with 4 ppg 20/40 mesh sand, and borate-crosslinked 35 lb/Mgal guar gel with 7.3 ppg 20/40 mesh sand. Test conditions cover a shear rate range of 60 to 120 sec -1 . In addition, the effect of an enzyme breaker on proppant settling rate under static conditions was studied. Tests were conducted with a fracture gap width of 0.375 in, under ambient conditions. Analysis of the dynamic condition indicates that proppant settling within the linear and crosslinked gels investigated is non-linearly dependent on shear rate. Under static conditions, linear gel slurries settled 40 to 50 times faster than the crosslinked gel containing breaker. It is also shown that an increase in shear rate induces proppant settling in borate-crosslinked 35 lb/Mgal guar gel while it reduces the settling rate in 35 lb/Mgal guar linear gel.

Experimental Investigation of Frictional Pressure Losses in Coiled Tubing

This paper presents an experimental investigation of tubular frictional pressure loss in coiled tubing and straight sections of seamed and seamless tubing. Fluids investigated include water, linear guar gum and hydroxypropyl guar (HPG), and borate-crosslinked guar gum and hydroxypropyl guar, under conditions typically encountered in many coiled tubing field applications. The equipment used includes a system of one 1000 ft and two 2000 ft coiled tubing reels that can be arranged to provide total coiled tubing lengths of 1000, 2000, 3000, 4000, and 5000 ft. The system also includes straight sections of seamed and seamless tubing with the same nominal diameter (1 1/2 in.) and wall thickness (0.156 in.) as that of the coiled tubing. The investigation focuses mainly on the effects of coiled tubing curvature, tubing seam, fluid pH and shear history on frictional pressure loss. Results obtained with water indicate that curvature as well as the seam inside the coiled tubing significantly affect the frictional pressure losses. Results obtained with various polymer solutions and gels, however, suggest that tubing curvature has a more significant effect on the frictional pressure losses than the tubing seam. Moreover, it is observed that for borate-crosslinked HPG the pressure gradient is dependent on both the fluid pH and the length of coiled tubing across which it is measured. However, for borate-crosslinked guar gum the pressure gradient is a function of pH and is not very sensitive to the length of tubing along which it is measured.

Proppant Transport Characterization of Hydraulic Fracturing Fluids using a High Pressure Simulator Integrated with a Fiber Optic/LED Vision System

Slurries of selected hydraulic fracturing fluids such as 40 lb/Mgal HPG linear gel and borate-crosslinked 35 lb/Mgal guar gel were evaluated to characterize their proppant transport properties using 6 ppg 20/40 mesh sand. In addition, fluids such as water, 40 lb/Mgal HPG linear gel, and borate-crosslinked 35 lb/Mgal gel were evaluated to determine their relative ability to remove the fluidized layer lying along the top of settling proppant bed and to subsequently erode the more compacted layers below. The experimental study utilized a unique high pressure parallel plate flow cell, simulating a downhole fracture, integrated with a vision system of fiber optic and Light Emitting Diodes (LED). The vision system was used to quantify proppant transport characteristics and to study various aspects of proppant transport including bank buildup. Selected hydraulic fracturing slurries were pumped into a high-pressure flow cell having a 0.375 fracture gap width. The slurries were pumped through 3000 ft of 1.188 in. ID coiled tubing, 500 ft of 2 in. ID heat exchanger, and a 2.75 in, inlet manifold that could be configured to represent a wellbore having various perforation arrangements. Slurries were pumped for a selected time at a fixed shear rate in the range of 60 to 120 sec -1 to observe the rate of bed sedimentation. These initial slurry injections were followed by various fluids and slurries to assess the degree of compaction within the previously deposited bed. Data analysis indicates that slurry sedimentation in polymer solutions follows a non-linear relationship with time at a fixed shear rate and that perforation configuration affects proppant transport, bed height growth, and proppant bed wash off near the wellbore. It is also shown that bed height growth is not directly proportional to shear rate. Results from the current study also show that the fluidized layer along the top of the proppant bed is easily removed by a clean crosslinked gel. However, erosion of the more compact underlying layers was found to depend on a number of factors. Among these factors are the type of slurry which formed the deposit, the type of fluid or slurry being used in an attempt to erode the bed, and the total pumping time devoted to eroding the bed.

Hydraulic fracturing slurry transport in horizontal pipes

Horizontal-well activity has increased throughout the industry in the past few years. To design a successful hydraulic fracturing treatment for horizontal wells, accurate information on the transport properties of slurry in horizontal pipe is required. Limited information exists that can be used to estimate critical deposition and resuspension velocities when proppants are transported in horizontal wells with non-Newtonian fracturing gels. This paper presents a study of transport properties of various hydraulic fracturing slurries in horizontal pipes. Flow data are gathered in three transparent horizontal pipes with different diameters. Linear and crosslinked fracturing gels were studied, and the effects of variables--e.g., pipe size; polymer-gelling-agent concentration; fluid rheological properties; crosslinking effects; proppant size, density, and concentrations; fluid density; and slurry pump rate--on critical deposition and resuspension velocities were investigated. Also, equations to estimate the critical deposition and resuspension velocities of fracturing gels are provided.

Borate-Crosslinked Fluid Rheology Under Various pH, Temperature, and Shear History Conditions

The oil and gas industry has now long been aware of the fact that the rheological characterization of crosslinked fluids under field simulated conditions is very difficult. Particularly, the testing of borate-crosslinked fluids under realistic conditions for better understanding of their rheological behavior, is very complex. For this purpose, the Gas Research Institute (GRI), the U. S. Department of Energy (DOE), and the University of Oklahoma jointly established the Fracturing Fluid Characterization Facility (FFCF) which includes a pilot-plant scale, high temperature, high-pressure simulator (HPS) coupled with a fluid pre-conditioning system. Employing the field-size equipment of the FFCF and field-simulated procedure, various borate-crosslinked fluid formulations are tested and evaluated for their rheological responses. This paper presents the results of pre-conditioned fluid rheology tests conducted with borate-crosslinked Guar and HPG gels employing the HPS over a range of pH from 9 through 11, subjected to varying levels of shear history and temperature. The network structure of these crosslinked fluids at any time is dependent on the current shear state, past shear history, and borate ion concentration which in turn depends on pH and temperature. Even though the effect of pH and temperature on the borate ion concentration is relatively well understood, the characterization of the shear state of the gel at field conditions is still in its infancy. In this investigation, test fluids are subjected to all conditions, to the maximum degree practical, experienced by a fracturing fluid in the field, including its formulation in surface equipment, its injection through tubular goods and perforations, and its flow and heat-up in the fracture. Besides the effects of polymer type, temperature, and fluid pH, the effects of shear history on the rheology of borate-crosslinked fluids are also investigated. The presented results show dramatic effects of shear history on the rheology of borate-crosslinked gels which have been ignored in previous investigations. Certain gel formulations corresponding to specific pH and temperature conditions, were found to be shear history insensitive. Furthermore, these shear history insensitive formulations were also found to exhibit an optimum viscosity which was independent of temperature over the range of ambient to 185 °F.

New Perforation Pressure Loss Correlations for Limited Entry Fracturing Treatments

Estimating the perforation pressure loss is an essential part in the design and analysis of hydraulic fracturing treatments. Accurate determination of perforation pressure loss for the rheologically complex fracturing fluids being used today can best be achieved through experimental study. Investigation of the perforation pressure loss for linear polymer solutions, crosslinked gels, and fracturing slurries has been conducted at the Fracturing Fluid Characterization Facility (FFCF) since 1994. Using the data acquired from these experiments, new correlations are developed to estimate the coefficient of discharge used in the orifice equation that governs the perforation pressure loss. The correlations can be used to accurately predict the coefficient of discharge for linear polymer solutions and titanium/borate-crosslinked gels. In addition, the slurry correlation can be utilized to determine the dynamic change in the coefficient of discharge for fracturing slurries due to erosion. The presented correlations are developed such that they can easily be incorporated into current fracturing simulators.

Tests Confirm Operational Status of a Large-Slot Flow Apparatus for Characterizing Fracturing Fluids

A large-slot flow apparatus for investigating hydraulic fracturing fluid flow and proppant transport phenomena has been jointly developed by the Gas Research Institute (GRI), the Department of Energy (DOE), and The University of Oklahoma. A series of tests were performed to confirm the operational readiness of this apparatus to perform the many functions for which it was designed. This paper describes results obtained from the verification tests, particularly in terms of the proper functioning of instrumentation and control systems under a wide range of operating conditions. Tests performed were: flow of crosslinked gels, high-temperature (200 and 225 F) flow tests, high-pressure (1,000 psi) flow tests to confirm proper operation of a unique throttling valve, effect of pressure on perforation pressure loss, and dynamic fluid-loss tests with permeable facings at 1,000 psi.

Fracture Flow Capacity - A Key to Sustained Production after Hydraulic Fracturing

The objective of a hydraulic fracturing treatment is to place a conductive fracture deep into the producing formation. Fracture flow capacity, or ability of the propped fracture to conduct fluids, is one of the most important aspects of the treatment. Several laboratory test procedures have been developed to evaluate proppants under a wide range of closure pressures and concentrations. As well depths increase, it is necessary to adjust proppant systems to compensate for additional loads under producing conditions. Recently, success has been encountered using mixtures of sand and high strength proppants in deep formations in Northern Louisiana, East Texas, and Oklahoma. Information provided in this work will offer guidelines which should be considered for any fracturing treatment using propping agent. Fracture flow capacity of the fracture system is of utmost importance currently because many marginal zones are being completed and fractured. Sustained production increase is also very important. Laboratory and field application data indicate proppant systems initially selected have an influence on well production behavior throughout the producing life of the well.