Dr. Ali Daneshy Daneshy Consultants Int’l Mitigating Frac-Driven Mechanical & Production Damage to Horizontal Wells Dr. Ali Daneshy Daneshy Consultants Int’l

Introduction The oil and gas industry has recognized that certain level of frac-driven interactions (FDI)s within and between horizontal wells is unavoidable for economic production from these reservoirs. The main point of discussion is NOT to eliminate FDIs, but to reasonably mitigate their damaging effects Better understanding of the types and effects of these interactions is essential for effective mitigation of their negative effects

Definitions Active well. This is the well that is being fractured Passive well. This is the well offset to the active well that has been fractured previously Active fracture(s). Fracture(s) that is/are being created at the time under consideration Passive fracture(s). Existing closing/closed fracture(s) created previously

Fracture Types For the same fluid volume, planar fracture will be the longest, and complex fracture the shortest. How does this match our field observations? An “off-balance” fracture has a nearly planar path, but includes shear fractures and branches along its length Off-balance Planar Complex

Detecting Inter-well FDIs Before fracturing Pressure changes detected while running DFIT During fracturing Pressure changes detected in offset wells during active fracturing Post fracturing Production interference Wellbore damage Frac fluid tracer recovery from offset wells

Consequences of Inter-well FDIs (While Frac’ing Damage) Most of the damage caused by frac-induced interactions occur during the fracturing operations. These can be divided into two general categories; Production loss Wellbore damage Permanent (liner rupture, production equipment failure, ..) Temporary (well debris, mostly proppant) Requires costly repair and clean-up operations and missed production days

Sheared Liner in the Passive Offset Well (Whitfield et al (2018).

Recovered Sludge from a Passive Offset Well Field case history (from Whitfield et al (2018).

Damaged Liner in the Passive Offset Well Field case history (from Whitfield et al (2018).

Types of Inter-well FDIs (Fracture Shadowing) Fracture shadow area Passive Well Active Well Old passive fracs Active frac Pressure inside the active extending fracture compresses the formation around it and causes the fluid pressure in the passive fractures in the shut-in passive well to increase Higher fluid pressure in the active fracture is also transmitted through the reservoir, but it has a much smaller contribution than the elastic formation compression

Fracture Shadowing (Field Case History)

Types of Inter-well FDIs (Frac/frac Connections) Manifestation of these types of links include; Fluid migration into the passive well causes its pressure to increase Tracers mixed with frac fluid are detected in the passive well produced fluid samples These links can be divided into two groups; Temporary frac/frac connections Fluid migration into the passive well ends shortly after the end of active frac stage The connection between the two fractures is through a minor secondary link Long term frac/frac connections Fluid migration between the two wells continues for some time beyond the end of pumping Connection between the two wells is through a more conductive link

Temporary Frac/frac Connections (Case History) Larger pressure changes than in fracture shadowing Small pressure increase rate indicates low conductivity link Fluid exchange ended close to the end of pumping

Long-Term Frac/frac Connections (Case History) Passive/monitor well pressure increases of several tens or hundreds of psi Hydraulic link between the two wells continues beyond the end of pumping that stage This type of interaction may be repeated several times in different stages, but may not be present in every stage

Effect of Depletion on Inter-well FDIs Depletion causes a drop in reservoir pressure around the producing well Reduced reservoir pressure reduces the magnitude of the in-situ principal stresses Since hydraulic fractures propagate along the path of least resistance, the reduced principal stresses attract approaching hydraulic fractures The net effect is that active fractures in an infill well grow longer in the direction towards an existing producing well Classifying wells based on their depletion status helps clarify fracture growth pattern and preference

Definitions Primary well. This is the first well drilled for production in a given area which is not within the depletion zone of another well. This well is sometimes referred to as the “parent well”. Infill well. This is a well drilled for production of the same area which is located within the depletion zone of the primary well or an earlier infill well. This type of well is sometimes called “child well”. A well drilled for production offsetting a primary (and/or other existing infill well(s)) whose targeted pay interval overlaps the primary or other existing infill well(s).

Past Attempts at Mitigation Do Nothing This was the initial industry response before potential negative side effects of frac- induced interactions were observed and recognized by the industry Occasional recovery of liquid tracers mixed with the frac fluid from offset passive wells Production damage in offset wells (reduced oil and/or increased water production) Expensive legal issues if production damage occurred in neighbor’s well

Past Attempts at Mitigation Shut-in the Offset Well The expectation was that shutting the offset well will prevent/reduce frac fluid migration inside it. At the recommendation of technical experts some operators decided to record the pressure of the shut-in wells during active fracturing operations. These measurements showed that frac-induced interactions were much more pervasive than recognized, and their type varied during different frac stages Shutting the offset well and recording its pressure was mandated by regulations in Canada The recorded data also provided the information needed by frac experts to analyze the problem and determine its causes The process was found ineffective for avoiding frac- induced interaction

Past Attempts at Mitigation Concurrently Produce the Offset Well (Frac & Flow) High pressure build-ups caused by frac- induced interactions was thought to be the cause of severe casing/liner damage experienced in some offset wells. Continued production was intended to keep their pressure at low levels In practice, this action was found to further aggravate the problem and even resulted in having to abandon some of these wells

Past Attempts at Mitigation Low Rate Pre-loading Fluid was injected at low rates into the offset well, either shortly before or while fracturing of the active well was in progress. The intent was to compensate for pressure and stress reductions caused by fluid production and reservoir depletion Low rate injections required lower and more manageable operational and material costs The small volumes of injected fluid were insufficient to make up for much larger volumes of produced fluid In practice, this action did not eliminate frac-induced interactions, but seems to have reduced their intensity and occasionally the damage

Past Attempts at Mitigation High Rate Pre-loading This system was the logical progression of the partially successful low-rate pre-loading. It resulted in less wellbore mechanical damage and production loss compared to low-rate system It also required higher operational and material costs, limiting its application to more productive reservoirs

Past Attempts at Mitigation Re-fracturing the Offset Passive Well This system was the logical progression of the success of the high rate pre-loading. Its variations included; Concurrent re-fracturing of the offset well together with active well fracturing Re-fracture the offset well shortly before active well fracturing It requires higher operational and material costs, limiting its application to more productive reservoirs

Past Attempts at Mitigation Use Diverters to Reduce Interaction Intensity Intent is to create a low conductivity barrier inside the active fracture in order to reduce volume and intensity of frac-induced interactions Several cases of its use and partial success reported in the literature Lower number of high intensity interactions has been cited as sign of success So far, field experience with use of diverters for avoiding wellbore mechanical damage and production interference has been inconclusive

Evaluating Potential for Well Damage Caused by Inter-well FDIs Recording and analysis of pressure increase caused by FDIs can indicate the likelihood and severity of the damage that can be caused by them. With proper preparation, this analysis can be performed real-time.

Example Case History

Example Case History Computations First interaction data ΔP = 344 psi Δt = 77 min Using known well information; Migrated fluid volume ~ 177 ft3 & q = 2.3 ft3/min Similarly, using the data from the second measurement; ΔP = 27 psi Δt = 84 min Migrated fluid volume ~ 13.8 ft3 & q = 0.16 ft3/min These computations indicate that the first interaction was caused by frac/frac connection and the second one was more likely caused by fracture shadowing

Completion Steps for Mitigating Frac-Induced Inter-well Interactions in New Developments Drill and construct the wells before fracturing operations This avoids harmful interactions between new wells and existing fractures Pad drilling is a very effective tool in this regard Fracture all wells before starting production in any of them This avoids the negative effects of reservoir depletion on fracture growth pattern Zipper fracturing or simul-fracturing All wells are basically pre-loaded while fracturing

Fracture Propagation in Zipper and Simul-fracturing Stress shadowing slows fracture growth in the middle overlapped zone in favor of outside growth

Diagnostic Use of Inter-well FDIs Careful examination of fracture interaction results can provide very valuable information about the created fractures, including; Length Height Growth pattern (simple, off-balance, complex) Presence and effect of natural fractures Type and intensity of interactions with offset wells

Concluding Remarks Inter-well frac interactions are a necessary part of economic viability of unconventional reservoirs. We can increase fracture and well spacing to a point where these interactions are completely avoided. But this is likely to make production from these reservoirs uneconomical with today’s oil and gas prices. Good engineering means reducing the negative effects of these interactions and maintaining the production high enough to make the entire process financially viable!

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