Case Study: Mixing Proppant Sizes To Control Pressure-Dependent Leakoff

Abstract

One of the main factors affecting fracture treatment completion in the Cardium formation in North West Central Alberta is PDL. PDL is determined from minifrac analysis completed prior to the main treatment. PDL can be treated and controlled with a combination of increased fluid rates, additional 50/140 proppant (100 mesh) in the pad and increased gel loadings. Yet in certain areas even these precautions will not achieve successful placement of job design or effective fracture properties. Common practice is to design for 80 to 100 Tonnes of ceramic proppant into each of these reservoirs, with an additional 5 to 7 Tonnes of 50/140 proppant in the pad to control PDL. In most areas this works effectively for controlling the competing fracture leak-off during the treatment. In the Northwestern Alberta Cardium area this approach has not been as successful and has caused concern about total proppant placement. An alternative approach for controlling fluid leak-off during the treatment has been to mix in a small amount (usually 5% by weight) of 50/140 proppant with the ceramic proppant in the early stages of the treatments to control the fluid leak-off in these areas. Treatment design engineers recognize the fracture conductivity issues caused by mixing different mesh sizes. However, utilizing this approach has resulted in full job completion and this paper will discuss the specific treatment designs required to drastically limit fracture treatment screen-outs. Introduction As the Oil and Gas Companies continue to explore and complete wells, a majority of them are now finding that the new areas within the Deep Basin located on the western edge of the Western Canadian Sedimentary Basin (WCSB) are becoming more difficult and challenging to complete. Many of these wells are tapping into reservoirs that are both low in permeability and located in regions that are structurally complicated due to massive faulting and folding associated with mountain building. Given the very low permeability of the Cardium formation, hydraulic fracturing is required to achieve economic production rates. In the different regions in the WCSB where the Cardium formation is situated, and where each area varies in terms of hydraulic stimulation requirements, there have been a variety of approaches to formulating the appropriate stimulation design for these wells. In most of the areas, Pressure Dependent Leakoff, dilation of natural fissures (PDL), is a dominant treating characteristic of this formation which must be recognized and treated accordingly. In many cases, controlling the PDL (determined through mini-frac analysis) has been straightforward requiring either a small volume of a higher viscosity pre-pad 1–3, or the addition of 1 to 2 tonnes of 50/140 mesh sand 4, or both, to control the leakoff into the naturally occurring fissures 5. This, in turn, has resulted in job placement for most of these areas to be above 95%. In the Northern Deep Basin region, initially considered as another 'typical' Cardium completion, a low completion percentage (35%) showed that the initial design was found to be inadequate in controlling leakoff associated with the natural fractures present. Within this initial design, changes to the pad size, treatment rates, and concentrations did not drastically enhance the ability to fully complete the treatment. As a result, a novel and controversial approach was introduced to address PDL in the Northern Deep Basin region. This approach involved introducing a small amount of 50/140 mesh sand in the proppant stream at a relatively low volume (3%-5%) to enhance control of continual leakoff. Despite common agreement that adding smaller mesh sand drastically reduces proppant pack conductivity 6, the operator has continued to endorse this new design, based on a significant increase of total job placements from an average of 35% to a current completion efficiency of 100%. Background The Cardium formation was formed during the Late Cretaceous period of the Mesozoic Era, an estimated 88.5 million years ago. The Cardium formation extends from Waterton Lakes National Park (and the Canada - United States border) through Alberta and beyond to Dawson Creek, British Columbia Fig. 1. Although the formation degrades into shale in the southeast, the gross thickness of the Cardium can range from 15 to 125 metres.