Transport of Proppant in Hydraulic Fractures

Abstract

Abstract Prediction of proppant transport is important in both treatment design and post-treatment analysis. Recently, there has been a resurgence of interest in proppant transport as evidenced by the number of papers presented at SPE meetings. There are a number of factors that have been overlooked in these papers that we believe are of primary importance. This paper summarizes the work that has been done in the area of proppant transport in settling and non-settling fluids. We also present the results of analyzing convection in turbulent flow fracturing treatments. Introduction Since proppants were first used in hydraulic fracturing, the question of proppant placement in the fracture has remained, for the most part, unanswered. Prediction of proppant placement is important in fracturing treatment design and post treatment analysis and research work reported over the years has contributed greatly to our understanding of the proppant transport process. In this paper we will cover the major modes of transport and discuss the influence of fracture geometry on proppant placement. The Transport Process Thinking about proppant placement has been strongly influenced by assumptions made about fracture geometry. Early theoretical work on fracture geometry yielded a picture of fractures that were elliptical from top-to-bottom or end-to-end with a relatively well-bounded top and bottom. This led to a view of fractures with parallel or nearly parallel sides and a rectangular shape. Experimental proppant transport models were and still are, for the most part, based on these ideas. In many ways, it is the only practical approach to model building. As we will see, it can lead to rather naive view of the placement process. The taxonomy of proppant transport starts with a division of the process into two major groups based on fluid properties: non-crosslinked fluids where particle settling can be important and crosslinked fluids where it is not. Early proppant transport studies were primarily concerned with fluids in which it was thought that settling strongly influenced proppant placement. As in other areas of fracturing, work done by Perkin and Kern1 laid the groundwork for understanding proppant transport in non-crosslinked fluids and served as a basis for a number of later experimental models. The accepted view of proppant transport in fluids in which particle settling occurs at a significant rate is that settling is governed by Stokes type particle settling velocity, proppant concentration, and the fluid velocity in the fracture. Particles enter the fracture and are transported uniformly away from the entry point until they are deposited on the bottom of the fracture. Deposition results in the buildup in a bank of proppant that is limited in height by the velocity of the fluid across the top of the bank. Because the proppant bank is the most prominent feature of proppant transport by non-crosslinked fluids, these fluids are often referred to as "bank-building fluids." With crosslinked fluids, settling is not an issue. The proppant goes with the fluid. And thus, there are two distinct general transport mechanisms. We will begin with crosslinked fluids because transport in these systems is, in many ways, simpler. Crosslinked Fluids In transport by crosslinked or non-crosslinked fluids, the first thing to remember is --- nature is lazy. The fluids will always take the easiest route through the fracture. And with crosslinked fluids, the proppant goes where the fluid goes. There are two distinct wellbore configurations that have an influence on the final proppant distribution. If the perforated interval is short relative to the fracture height, flow into the fracture can be considered to be flow from a point source. While a more even distribution of perforations relative to the fracture height will provide a more uniform slurry distribution from top to bottom. Depending upon the number and location of perforations that actually accept fluid, local point source flows can be expected to influence the proppant distribution.