This study investigated selected problems associated with hydraulic fractures using gelatin to simulate poorly consolidated sediments. The main objective was find ways to surround a vertical well with hydraulic fractures and extend selected ones into the formation as wing fractures to set an outside gravel pack as an economical alternative to enhance hydrocarbon productivity.
A technique was developed to control initial hydro-frac orientation based on tensile strength, fluid inertia, and breakdown pressure. This, in conjunction with an appropriate shooting phase and hole orientation enable the appropriate space to set an outside gravel pack.
Introduction
The outside gravel pack technique was tried for the first time by THUMS and ARCO E&P Technology at the Wilmington Field in 19971, where resin coated sand was injected through 180° phase perforations oriented perpendicular to the minimum horizontal stress with the purpose of surround a well, creating a "halo effect" and extending it far into the formation as two wing fractures (idealized in fig. 1), to enhance production by leaving the well's inside free of gravel2.
The well produced 250 barrels/day three months with restricted production, then started to produce sand (up to 6%), forcing to shut it down3. Because of this unfortunate situation ARCO E&P Technology funded a Master Thesis4 to study ways to improve the outside gravel pack technique, which was done using common gelatin as an inexpensive physical model material to simulate soft sediments layers.
Gelatin had being used successfully since 1957, when Hubbert and Willis5 did a class experiment showing that hydraulic fractures propagate perpendicular to s3, from there, gelatin had being used to study and simulate geology processes6,7 or helped understand stresses that affect engineering designs such as tunnels8 or open pit mines9,10.
For this project we used transparent gelatin from Knox company, which was prepare in two different concentrations:A normal concentration following the instruction from the manufacture, which was used as a pay zone layer, andadding twice the amount of granular gelatin set by the manufacture for the same amount of water, to simulated barrier layers.
For the hydraulic fractures, was used colored tap water injected through a simulated well (1/4 inch copper pipe) pierced with horizontal holes (1.15 mm) at the same depth simulating gun perforations used by the oil industry11.
The main objective was find ways to create with hydraulic fracturing in gelatin the halo effect and wing fractures (fig. 1) at two different horizontal stress environments:When horizontal stresses were equal (s2 = s3) andwhen horizontal stresses were different (s2 ? s3).
Dynamic and static tests were performed with the two types of gelatin (pay zone and barrier layers) to obtained their elastic properties (Young's Modulus and Poisson's ratio) so these experiments can be scale to real field cases12.
The use of hydrajetting for perforating of wells has been commonplace since the 1960s. During those early years, wells were relatively shallow, and jetting success was consistently demonstrated. However, as wells became deeper, where rock formations tend to be harder, hydrajetting performance became less dependable; subsequently, stimulation failures more often occur as a result of the lack of fracture initiation.
To remedy this situation, a series of tests were performed to define new best practices for hydrajet perforating of hard rock under high ambient pressure environments. Various rocks were subjected to these tests, which were performed using different jetting pressures and different abrasives. The perforation surfaces were then dissected and evaluated using photographic and chemical means. Further assessments were then made to determine what actually occurred during the hydrajetting process.
This paper discusses various test results, and new constraints for hydrajetting are defined and presented.
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