Comparison of Fracture Conductivities from Field and Lab
Authors: Zhou Desheng , Zhang Gang , Ruan Min , He Anwu , Wei Dengfeng 
Published: May 2016

American Petroleum Institute (API) and International Organization for Standardization (ISO) provide short-term and long-term proppant pack conductivity tests in lab. Those are the standard in testing proppant pack conductivity and are widely used in the industry during fracture design. However, the standard results are far from actual fracture conductivity as many factors are not included in the standard tests.

By comparing the environment difference between the standard lab conductivity and actual fracture conductivity, the paper summaries the most possible factors impacting the actual fracture. Those impacting factors are categorized into four types in the paper: proppant filling reduction, Proppant filling reduction, Porosity reduction, and Fluid flowing reduction.

Well testing and production data analysis are used in explaining fracture conductivity, but the accuracy is unsatisfied for reservoir, perforation and turtorasity, and geology uncertainties. In-situ measurement is needed to obtain actual fracture conductivity.

Introduction

Hydraulic fracturing is a major technique in enhancing petroleum recovery reserves and daily production, especially for low-permeability reservoirs. The first fracturing treatment was introduced to the industry in 1947 in the Hugoton gas field in an effort to increase well deliverability of four acidized limestone pay zones. Up to now, millions of fracturing treatments have been done.

The major propose of hydraulic fracturing is to create a long and conductive fracture to increase a well production. With the created fracture, reservoir fluids flow through the "tough way" in low permeability reservoir into the fracture, then flow easily through the "free way" in high permeability fracture. The enhancing mechanism of hydraulic fracturing is to increase the exposure area of reservoir to wellbore, to bypass the near-wellbore damage zone, and to change oil and gas flow pattern by creating a long and highly conductive flow path in the reservoir.

Proppant is used to hold the fracture width open. Depending on the closure pressure of the opened fracture, sand, ceramics, resin coated ceramics, bauxite, or their mixture is the commonly used proppant in the industry. Viscous fluid carrying the proppant is pumped deep into the created fracture. Sufficient quantities of proppant are required in the fracture to prevent the fracture from closing after the pumping is stopped. A highly conductive proppant pack in the fracture is paved.

The viscous fluid is commonly water-based polymer, oil-based polymer, or aqueous foams with additives including fluid-loss additives, breakers et al.. To carry the large amount of proppant, high viscosity fluid like linear gels or crosslinked fluid is used. After pumping, the viscous slurry will break back to low viscosity to flow back and leave a highly conductive proppant pack in the fracture.

A long and conductive fracture is one of the most critical parameters for hydraulic fracturing. The proppant pack conductivity is measured in lab following API RP 61 (American Petroleum Institute, 1989). However, the results from well testing and production data analysis often show a "bad" fracture comparing design to lab data. The "bad" fracture has much low conductivity if the fracture length is equal to that from design, or much short fracture length if the conductivity is equal to that from lab.