Pores to Predictions: Nanoscale Clues to Larger Mysteries
- Stephen Rassenfoss (JPT Emerging Technology Senior Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- December 2016
- Document Type
- Journal Paper
- 28 - 33
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The story of unconventional oil and gas technology development has been focused on fractures. The formula has been more stages, more sand, and more water, targeting the most productive spots.
The trend continues, with rising production and larger completions at a time when the oil price bust has slashed the cost of services and supplies. Even so, production from unconventional wells leaves 90% of the oil in the ground, raising the question, what is next?
Some have taken on the daunting task of trying to understand how oil and gas flows out of the tiny pores in ultratight rock that feed the fractures flowing to the well.
Compared to what it takes to effectively fracture rock and target sweet spots, the science behind fluid and gas flows through pore networks is a difficult slog deep into the unknown. It is a frustrating search, but that is where the oil is.
“The first part of the problem is that people started to say that oil and gas is stored in the fractures and that by fracturing a well you are simply exploiting that storage event,” said Richard MacDonald, the innovation, strategy and execution manager for EP Energy, which is among the companies looking deeper (Fig. 1). “We calculated what fractures will hold and that volume will last only a week or week-and-a-half of production. After 2 weeks, where is the oil from? The matrix.”
The matrix is a frontier, like the unexplored lands shown in maps made centuries ago during the era of exploration when those vast uncharted spaces spawned fantastic tales. These explorers are looking at explanations for production patterns that diverge from what they know.
“It is not something they taught us in school,” said Steve Jones, reservoir engineering advisor for Cimarex. “We are learning, figuring out what are the facts of life and the ground rules, the theory.”
Based on his experience at Newfield Exploration, he delivered a technical paper at the recent Unconventional Resources Technology Conference (URTEC 2460396) explaining why changes in the gas/oil ratio (GOR) produced from certain unconventional wells, which would be alarming in a conventional field, are the norm in the unconventional plays he saw in Oklahoma (Fig. 2).
Jones is among those working on field-scale problems. Others are doing pore-scale studies on this ultratight rock where standard terms such as reservoir and transient seem an odd fit based on the established definitions.
While an online dictionary defines a reservoir as a place where fluid collects, especially in rock strata, it offers a poor mental image of unconventional rock. There the oil in the rocks is scattered throughout pore networks with openings ranging in size from a large virus down to just a bit bigger than a DNA molecule. Spaces at the low end of the range are so confined they can alter the physical properties that control production, particularly the bubblepoint—the pressure at which the gas in oil escapes from the liquid.
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