A computer program has been developed to predict gravel placement effectiveness for a specific gravel-pack design. This program was developed as the direct result of recent studies using a large-scale, transparent, deviated wellbore model.
Earlier studies have addressed the problems associated with gravel packing deviated wellbores. Each of these studies has utilized wellbore models and circulatory techniques to illustrate the placement of gravel packs. Each also proposed various solutions to maximize the packing efficiency. A mathematical model was reported in these earlier studies which related wellbore condition and fluid properties to gravel placement effectiveness for a specific gravel-pack design. This most recent work was conducted in order to determine the accuracy of that math model prediction.
The accuracy of the computer predictions was evaluated by the use of a series of test gravel packs performed with the wellbore model. Throughout the testing, fluid density, fluid viscosity, particle size, particle density and concentration, percentage of flow loss, and casing I.D. were held constant. The pump rate, screen and tailpipe size, and angle of deviation were varied.
In each test predicted to produce a complete pack, 95 to 100% of the casing volume throughout the length of the screen was packed properly.
Presently, gravel packing is the most widely used method for controlling sand production from unconsolidated formations. The popularity of gravel packing can be attributed to the reliability and efficiency of this service. The procedure requires placing a wire-wrapped screen or slotted liner in the wellbore through the producing interval, then completely packing the wellbore-screen annulus with gravel.
The initial success and ultimate lifetime of a gravel pack may be influenced by a number of factors, among these complete coverage of the screen is of primary importance. Screens or slotted liners exposed to the production of sand-laden fluids may be damaged, thus requiring an expensive workover to repair. In vertical wellbores, complete coverage of the screen is routinely accomplished. However, in highly deviated wellbores, a complete packing of the wellbores can be very difficult to obtain.
In recent years, a number of studies1,2,3 have been conducted to define gravel transport properties in deviated wellbores. Maly et al. documented the mechanism by which incomplete or complete packs occur by performing gravel packing tests in a transparent, deviated wellbore model. He found that two competing flow paths exist within the wellbore (Figure 1) flow in the casing-screen annulus and flow in the screen-tailpipe annulus. To obtain a complete pack, Maly reported that flow in the screen-tailpipe annulus must be minimized.
If this condition was not met (i.e., if the resistance to flow in the casing-screen annulus was much greater than the resistance to flow in the screen-tailpipe annulus), premature bridging of the casing could result, producing an incomplete, unstable gravel pack. To maximize the flow in the casing-screen annulus and to avoid prematurely bridging the casing, Maly proposed using rubber baffles spaced along the tailpipe to increase the resistance to flow in the screen-tailpipe annulus.
Gruesbeck et al.2 illustrated with transparent wellbore models and field cases that highly deviated wellbores could be effectively gravel packed by evaluating each factor controlling gravel transport. These factors could be divided into two general categories: slurry properties and wellbore conditions. The properties of the slurry which influence gravel transport are the following.