Vertical communication between stacked horizontal wells and individual stage contribution from vertical wells is usually difficult to evaluate without spending a lot of money on isolation testing, pressure build-ups, or tracer surveys. A less expensive alternative is to collect isotopes of the produced gas and compare them directly to the formation gas at each depth. This method involves collecting isotopes of the mud gas during drilling to establish a baseline for the gas signature from each vertical horizon in a well. Provided enough variation exists between the horizons, this process can help determine zone contribution or well communication between stacked and staggered laterals. This methodology was used by Apache in the Delaware Basin to understand drainage and communication between stacked laterals in the Bone Spring and Wolfcamp.
Evaluating well communication is important because it influences future well planning and the completion design of unconventional wells. In the Permian Basin, there are multiple landing zones operators exploit for hydrocarbons and depending on the assumed drainage area by each well, companies stack and/or stagger wells to maximize development on their acreage. Interwell communication is common in pad development and it occasionally leads to positive results, but more often it negatively affects reservoir depletion. The negative results could be that reserves are shared between two wells or that less complexity occurs during hydraulic fracturing due to depletion causing undesirable production results. Interwell communication is not always easy to identify by analyzing just the initial rates and pressures. Even if initial rates and pressures indicate interwell communication, this does not necessarily translate into long-term interference.
Current Methods to Test Zone Interference
Methods considered to evaluate well drainage and test for zone communication are chemical tracers, micro-seismic, pressure build-ups, and produced fluid and gas comparisons, with each having its own pros and cons.
Chemical tracers are used to understand contribution along the lateral and frac hits between wells. These chemicals are pumped during different frac stages and usually consist of a combination of oil and water tracers. The water tracers bind to the frac water and indicate if the frac water has reached a reservoir that is being depleted by a nearby well. The oil tracers are oil soluble and hydrophobic which cause them to bind to the oil they contact in the reservoir. These tracers also indicate if a well fraced into another well and how long that connection lasts. Their limitation is they do not identify where a well is draining from, only what the frac fluid contacted.
