Sludge and Emulsion Prevention During Matrix Stimulation of an Ultra Low API Crude Oil Bearing Heterogeneous Limestone Reservoir for Production Enhancement
Authors: Amin S. , Saeed M. , Elmoneim H. , Yousuf A. , Imran M. , Shahzad K. , Sarbast M. , Zanaty O. E. , Taher Sherwan I. , Abdulrehman Farhad , Awdal Abdullah , Qadir Diary , Larder Jerry 
Published: May 2024

Abstract
Matrix stimulation is widely used for removing near wellbore damage and optimizing production from a wellbore. Understanding of the reservoir formation and fluid properties, play a key role in the success of a matrix stimulation treatment. Embracing the challenges posed by the nature of reservoir fluid is critical to devising a strategy for the matrix stimulation treatment. The selection of treatment fluid becomes a challenge in formation containing low API crude oil prone to causing sludge and emulsion with foreign fluids.
The subject well X was an exploratory well drilled to target the deeper prospects of a producing field. During drilling, losses were observed in the pay zone. Open hole logging confirmed deep damage in target zones. Multiple connected fractures were identified during log interpretation, however no flow from the reservoir was observed at surface during drill stem test (DST). Heavy oil was expected from the reservoir and ultimately downhole fluid sampling was performed to recover reservoir fluid sample. Recovered sample had tar like properties with the crude API of less than 10. Recovered sample was tested in lab for its sludge and emulsion tendencies towards the stimulation systems. It was determined that the reservoir fluid had tendency to form heavy sludge and emulsion in the presence of in-organic as well as organic stimulation systems. Moreover, the sludge and emulsion created was extremely difficult to break. Matrix stimulation treatment had to be modified to avoid damage. Matrix stimulation treatment consisting of a combination of hydrochloric and formic acid; reinforced with anti-sludge agent, non-emulsifying agent and mutual solvent; was designed after meticulous lab testing to carry out the matrix stimulation. Bull-heading of treatment fluids at maximum rate within the reservoir and surface pressure limitation was chosen as fluid conveyance mechanism, to provide effective diversion of treatment fluids and ensure uniform treatment.
The results of matrix stimulation treatment were very promising. Matrix stimulation was performed through DST string and after the treatment DST string was pulled out of hole. A completion string with electrical submersible pump (ESP) was run in hole (RIH) to flow back the well and carry out well testing. An appreciable improvement in crude oil production rates was observed. Furthermore, no sludge and emulsion were created in the reservoir. Well testing data interpretation also confirmed the success of matrix stimulation treatment.
This paper illustrates the strategy for designing and executing matrix stimulation treatment of heavy oil reservoirs whose formation fluid is susceptible to creating sludge and emulsion with matrix stimulation treatment systems. This type of stimulation technique was first of its kind, employed to avoid damage to the reservoir by stimulation systems. The results of this matrix stimulation strategy were exceptional and it can be referred to while dealing with similar challenges.