Bernard Tremblay

Imaging of sand production in a horizontal sand pack by X-ray computed tomography

A laboratory experiment was performed to better understand how sand production can increase heavy oil recovery. A horizontal sand pack with an orifice at one end modeled the production of oil and sand into a perforation in a vertical well. The sand pack was scanned using X-ray computed tomography (CT). The CT images revealed that a high-porosity channel (wormhole) formed in the pack while sand was produced. The wormhole followed regions within the pack where the porosity was higher, and, consequently, the unconfined compressive strength of the sand was lower. This experiment suggests that wormholes will form within the weaker sands of a formation. The development of these high-permeability channels increases the drainage of the reservoir, which leads to higher oil recovery.

Simulation of Cold Production in Heavy-Oil Reservoirs: Wormhole Dynamics

Cold production is a recovery process used in unconsolidated heavy oil reservoirs in Alberta and Saskatchewan, Canada. In this process, sand and oil are produced together under primary conditions. Oil production rates can typically increase by one order of magnitude when sand is produced. The production of sand into a perforation in a well was modelled experimentally using a horizontal sand pack. Heavy oil flowed through the sand and out the orifice at one end of the pack. The pack was imaged using an X-Ray CT scanner. A high porosity channel or wormhole was observed to develop in the sand pack above a critical pressure gradient at the orifice. The wormhole developed in the higher porosity region of lower cohesive strength and broke through to the inlet. The sand cut was 44% (by volume) as the wormhole was developing. When the wormhole broke through to the inlet, the sand cut declined sharply (power law function of time). CT images of the sand pack showed that the loose sand within the wormhole started to be scoured evenly at the top of the channel at this time. The experiment confirms previous assumptions that wormholes can develop in un-cemented oil sands leading to high oil production rates due to greater drainage of the reservoir. The experiment indicates that the weaker part of a sand formation (lower cohesive strength) may be more susceptible to wormhole development. Field observations of the sand cuts of wells on cold production are consistent with the experimental observations. The decline in sand production in these wells, from typically 40% during the initial stage to approximately 3% over a few months, suggests that the main wormholes have stopped growing. The residual sand cuts are postulated to be due to the scouring of the sand within the wormhole.

A Wormhole Network Model of Cold Production in Heavy Oil

High permeability channels (wormholes) are believed to be generated, starting from the wellbore and propagating into the reservoir, during the initial phase of cold production. The d evelopment of wormholes drastically enhances oil production. Understanding the wormhole development pattern, therefore, is critical to the modelling of the fluid flow behaviour and r ecovery rates in the cold production process. We propose that the wormhole growth can be described by the probabilistic a ctive walker (PAW) model, which is a generalisation of the classical random walk model. This simple description may lead to an improved understanding of: the mechanisms i nvolved in cold production, the fluid flow in the area where wormholes are formed (the wormhole zone) and field production data. Previous experimental and theoretical studies ind icate that the wormhole diameter may be a function of distance from the wellbore. We assume that this function follows a power law, slowly decreasing with increasing radial distance. We calculated the mobility of a slurry of sand and oil through the wormhole network. This mobility was used to calculate oil and sand production rates. Furthermore, we related the maximum size of the wormhole zone and its expansion rate to the sand production data in a typical cold production oper ation. This relationship can be useful in determining well spacing. This wormhole network model can also be useful as a tool for analysing field data and for the development of field scale numerical simulations of the cold production process. Cold production and wormhole development Cold production is a non-thermal primary heavy oil recovery process in which sand production is encouraged. It is believed that high permeability channels are formed during cold pr oduction. The main cause of wormhole formation is believed to be the flux of fluids through unconsolidated sand. This flux exerts a drag force strong enough to overcome the forces that hold sand grains together. The sand grains are transported along the wormhole. This kind of flux induced erosion pro cess is local at the wormhole tip as suggested from X-ray co mputed tomography images 1-5 of a wormhole as it grows in a sand pack. In these experiments, the porosity of the sand su rrounding the wormhole tip did not appear to be disturbed. Along the length of the wormhole, the porosity was roughly constant, around 55%. In addition, it decreased sharply, from 55% to 34%, as the wormhole tip was reached. 3 The dimensions of the pixels used in the reconstruction of the CT images were 1.5 mm by 1.5 mm by 8 mm long. In other words, instead of a collapse over a large area, this erosion process causes the surface of the matrix to successively erode at the tip of the wormhole. Experimental studies

Modelling of Sand Transport Through Wormholes

One of the key mechanisms in the cold production (CHOPS) process is the development of high permeability channels (wormholes), which increase the access to the reservoir. In order to model the growth of wormholes in the field, it is necessary to predict the flow of sand along the wormholes. The laminar flow of sand and oil through an open channel within a wormhole was modelled using an analytical Bingham Mohr-Coulomb model. This model was tested by comparing its predictions to oil and sand flow rate measurements obtained in pipe flow experiments reported in the literature. The model for sand transport was used in developing a field wormhole growth model.

Mechanisms of Sand Production Through Horizontal Well Slots in Primary Production

Cold production in unconsolidated heavy oil reservoirs, where sand production is encouraged (CHOPS), has proven to be successful for vertical wells. However, the application of CHOPS to horizontal wells has been less profitable due mainly to excessive sand cleanout costs. Therefore, reducing sand cleanout costs by controlling sand production into horizontal wells while enhancing the surrounding permeability of the formation is an important factor in optimizing cold production from unconsolidated heavy oil reservoirs. This paper presents the results of an experimental investigation of the flow of oil and sand in the vicinity of a horizontal well under cold production. Specifically, the experiments physically simulated the flow of oil and sand into a slot in a horizontal well liner. The parameters studied include slot width and sand properties (morphology and grain size distribution). Experimental results suggest that sand production through horizontal well slots can be controlled, depending more on sand grain sorting than on grain morphology or average diameter. The sand cut had a tendency to be higher at the beginning of the sand production period and to decline with time. In most tests, the decline in sand cuts continued until no more sand was produced. Significant changes in the permeability and porosity were determined in the vicinity of the slot. The changes in the parameters were less significant away from the slot The largest fractions of the sand (bigger than 500 μm) have an important role in arch/bridge formation.

A Model for Sand Transport Through a Partially Filled Wormhole in Cold Production

We propose a simple model of flow in a partially filled wormhole, where layers of oil, slurry and immobile sand can co- exist. The slurry at the bottom of the wormhole is assumed to behave as a Bingham material, i.e., it yields when the shear stress exceeds the yield stress. The yield stress is assumed to increase; with depth due to the weight of the overlying sand. A Mohr-Coulomb equation is used to calculate the yield stress within the slurry and immobile sand layers. Linearized Navier- Stokes equations are solved to calculate the oil and mobile sand flow rates. Different characteristic oil and sand flow patterns are studied. Oil and sand flow rates through the wormhole are calcu- lated as functions of pressure gradient and rheological parame- ters. This simple model could be used to estimate sand transport in a partially filled wormhole, complementing a sand transport model for uniformly filled wormholes(4). These wormhole flow models can be incorporated into a field scale model for cold production.

A Device and Method of Determining the Rheological Quality of Gelled Fracturing Fluids

A method of determining the quality of cross-linked hydraulic fracturing gels in the field was developed at the Alberta Research Council. The method is based on the measurement of the pressure required to push the gel through an orifice. The measurement can be repeated several times in order to quantify transient changes in the rheology of the gel. The quality of the gel prepared in the field is quantified by plotting the extrusion pressure vs. shear rate at the orifice. This quality control technique was compared, for certain types of water-based and oilbased gels, to existing techniques based on shear rheometry. The comparison showed that the water-based gels we investigated could be characterized using the gel tester, but not with the existing technology. The oil-based gels we investigated could be characterized better using the existing technology. To order the full paper, visit https://doi.org/10.2118/02-05-05