Microseismic technology continues to pay its way as an exploration tool.
A buried array installed in the Williston Basin is helping Newfield Exploration Co. of Denver increase efficiency, according to company geologists.
The array took 41 days to install in early 2012 and covers 58 square miles, with 229 holes, or stations, spaced about 3,000 feet apart with geophones placed at 300, 250 and 200 feet, according to AAPG member and Newfield geologist Laura Johnson.
"The array has proved to be useful in mapping microseismicity during hydraulic fracturing in the Middle Bakken and Three Forks reservoirs," Johnson said.
"The primary target reservoirs are the Middle Bakken and Upper Three Forks. In our study area the Middle Bakken is approximately 35 feet thick and the Upper Three Forks is 30 feet thick. The vertical distance between two laterals in these reservoirs in our
area is approximately 70 feet," she added.
"This work has helped enhance recovery mainly by helping us understand how many wells we need to drill to economically develop our field area. We also believe that by frac-protecting offset wells we enhance recovery by getting more effective fracs on new
wells and protecting the existing wells," Johnson continued.
"We confirmed microseismic event trends with chemical and radioactive tracers as well as downhole pressure and temperature anomalies in offset wells," she said. "We used the results to understand hydraulic versus effective stimulation. We use variable fracture
geometries in our reservoir models rather than running a simple constant fracture half-length and height. This modeling is being used to help us understand spacing, driving how many wells we drill in each reservoir.
"We also used the results to design our completion order laterally and vertically when completing multi-well pads and near existing wells," she continued. "We now frac the well nearest the existing producer to build up pressure and frac protect the next
wells to complete."
Johnson said the company also refrains from drilling within 1,200 feet of a well that has been hydraulically fractured within the previous six months.
Challenges Continue
Three pilot areas were chosen: two designed to test infilling a drilling spacing unit with an existing well and production and the other as "relatively virgin, meaning minimal existing production. We knew we had both situations in front of us to develop,"
Johnson said.
The situation has not been without challenges.
"The induced fracture heights are poorly calibrated in our buried array data. We knew this parameter would have greater error with the buried array technology. The best way to constrain it would be to run borehole microseismic but we have not done that yet.
This has made it difficult to understand vertical stimulation geometries and effects between the Middle Bakken and Three Forks, our two main reservoirs," Johnson explained.
She said there is room to push the envelope further to "continue to understand how the reservoirs complete and interact and what we can do to best drive enhanced recovery from the reservoirs."
"I think a leading next step for microseismic is to tie the microseismic events to completion parameters to help us tweak pump rates, fluids, proppant," she said, "to better stimulate the reservoirs and get more oil out for less money."