Challenge of Acidizing Horizontal Wells in Tight Carbonate Reservoirs - Weak Acid and Non-acid Alternates
- Sameer Punnapala (ADCO) | Maryam Abdul Rahman (ADCO) | Sanjay Misra (ADCO)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Petroleum Exhibition and Conference, 10-13 November, Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2014. Society of Petroleum Engineers
- Tight carbonate reservoirs, Stimulation, Chelating agents, Wormholes, Horizontal wells
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Horizontal wells have become a norm in ADCO for effective and efficient reservoir recovery. At present ADCO is embarking on development of tight carbonate reservoirs where permeability ranges from 0.2 – 10 mD. While carrying out a review of the 214 acidization jobs conducted from 2005 to 2013 in two of ADCO’s fields, it was found that conventional acid stimulation with 15% hydrochloric acid in horizontal wells in tighter parts of the reservoirs did not seem to provide expected results. Use of selective stimulation in segmented completion and diversion also did not yield the desired results. The reaction rate of 15% hydrochloric acid is too fast to create effective wormholing when tightness of the reservoir exceeds certain degree. A thorough survey of market and the published literature was carried out to analyze the root cause of the poor performance. Wormholing is the desired mechanism for carbonate stimulation, which is governed by the reaction kinetics and rate of transport. Optimal wormholing is obtained at a right balance between these two mechanisms. Literature is abundant with references on use of weak acids and non-acid stimulation chemicals like acetic acid, chelating agents and nano emulsions for stimulation of carbonate reservoirs. These chemicals have an advantage over 15% hydrochloric acid in terms of wormholing potential owing to slower rate of reaction. The study revealed that in a field, permeability and length of treatment are the main parameters in deciding the optimal stimulation chemical. A correlation was developed based on the kinetic parameters of various chemicals and field experience which can act as a guide in selecting the right acidization formulations. Such correlations will go a long way for ADCO in acid stimulation of horizontal wells in tight carbonate formations.
Oil well stimulation is desired to improve productivity of a well by removing the near wellbore damage. In vertical wells, a positive skin pertains to reduction of effective wellbore radius thus sacrificing the reservoir contact. It is very important to note that in long horizontal wells, wellbore radius does not represent the reservoir contact rather it is replaced by the horizontal length (Joshi, 1991). It can be simply said that horizontal well itself represents a limiting case of finite conductivity fracture. Thus in the case of a homogeneous reservoir, if vertical permeability is sufficient, the horizontal well is supposed to drain the reservoir. The primary aim of stimulation in such case becomes removal of near wellbore formation damage.
Two of the ADCO’s fields are entering a phase of development involving tight carbonate reservoirs. The field development is envisaged to be done by drilling 3000 to 5000 ft. long horizontal wells. 15% HCl is the conventional acidization formulation for these fields. A review of 214 conventional acidization jobs carried out in these fields from 2005- 2013 was conducted. It revealed that HCl stimulation did not yield the desired results in horizontal wells. Selective stimulation in segmented completion and by the use of diversion also did not change the situation. In view of this, ADCO embarked on an extensive market/literature survey to conduct a root cause analysis. The study revealed that weak acid and non-acid chemicals like chelates (EDTA, HEDTA, DTPA GLDA etc) and nano emulsions offer a viable alternate. Based on the study, using kinetic and transport parameters of different chemicals and core flood studies cited in the literature, a correlation was developed to correlate formation permeability and length of treatment with effectiveness of different stimulation recipes. This correlation was validated by the results obtained from the acidization review and a reasonable matching was obtained. This paper sets out the basis for wormholing phenomenon as described in the literature and establishes as to how kinetics of diffusion alone can be applied to estimate ideal wormholing. Based on this premise the said correlation was developed and validated.
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